Epdlib

Fix issue 15 - "unknown module" when display = "HD" and no vcom value set

Add option to force all blocks in a layout to 1bit mode. TT Fonts are rendered with anti-aliasing in all modes except for 1bit mode. Anti-aliased fonts display poorly on 1bit screens with extremely jagged edges.

**Layout**

* `Layout` objects now support boolean property `force_onebit`
- When set to `True` all blocks are forced to `mode = '1'`

rewrite of `Screen` module to fix unclosed SPI file handles
**Screen**

* `initEPD()` method is now depricated and no longer needed; this can be removed from the code
- displays are automatically woken prior to write/clear and put to sleep after write/clear
- at your own risk, use `writeEPD(sleep=True)` to keep display awake after write. For non-IT8951 boards, you **must** manually call `epd.sleep()` prior to the next write/clear event to ensure the SPI file handle is properly closed.
* `writeEPD()` and `clearEPD()` methods now handle all init, write and sleep operations automatically
- it is no longer necessary to call
* added convenience static method `list_compatible()` to print list of non-IT8951 boards

rework of `Block` and `Layout` modules to fix padding and text scaling

**Block**

* parent `Block` class now consumes any unknown `kwargs`
- this makes it much easier to pass any values from a `Layout.layout` obj.
* rewrite of text scaling to provide a cleaner image and crisper font rendering
* add `align` argument to `Block.TextBlock` for aligning text
* fixed various padding issues (again)
* Switched to using `multiline_text()` method of ImageDraw for better leading and line spacing

**Layout**

* Remove scale_x, scale_y from layouts
- this feature was not as useful as it appeared and is a pain to maintain
* update private methods:
- `_set_image` now `_set_blocks`
- removed `_check_keys`
- `_scalefont` minor updates
* default values for expected/necessary `Layout` and `Block` kwargs are now stored in `constants.layout_defaults`
* add property `mode` "L" when *any* block is 8bit "1" when all blocks are 1bit

**Screen**

* now supports IT8951 based panels
* `Screen.epd` object is now supplied as a string, not an object
- use `epdlib.Screen.list_compatible_modules()` to show compatible waveshare_epd panels
- use `Screen(epd='HD', vcom=[your IT8951 vcom (see ribon cable)]` for IT8951 based panels
* `Screen.writeEPD()` now supports partial refresh on IT8951 panels for partial refresh of black & white pixels
- use `writeEPD(image, partial=True)`
- partial refresh does not affect gray pixels

**Block**

* now supports 8 bit grayscale images, text and background on IT8951 based panels
- previously 8 bit images were downscaled to dithered 1 bit images; this is still the case on 1 bit panels
- use `Block.mode = "L"` for 8 bit support within a block
* now supports 8 bit fill and background on IT8951 panels
- use `Block.background=[0-255]` and `Block.fill=[0-255]` 0 is black, 255 is fully white
* padding now fully supported in all block types
- previosuly only worked properly on `ImageBlock` objects

**Layout**
* now supports the following features when specified in the layout dictionary:
- `'fill': 0-255` specify text fill (8 bit gray 0 is black)
- `'bkground: 0-255` specify background color (8 bit gray 0 is black)
- `'mode': "L"/"1"` specify block image mode "L" 8 bit, "1" 1 bit

**Screen.Screen()**

* add property: rotation(`int`): rotate the output image by 0, 90, -90 or 180 degrees

**Layout.Layout()**

* fix typos in example layout docstrings