Circuitpython-nrf24l01

Changes
- verified capability to communicate with an Arduino via the nRF24L01. Added an example as proof.
- changed CE pin to use `board.D4` in the examples as this pin is available on most CircuitPython MCUs and the Raspberry Pi doesn't use it for any special functionality.
- consolidated examples for both ItsybitsyM4 & Raspberry Pi into the examples folder (no more duplicate examples nor sub-folders). Also renamed the examples to suit `circuitpython-build-tools` changes.
- this also reverts back to a proper Python package structure for future development of additional modules. Note the code in `__init__.py` as it preserves backward compatibility with the old import statements (using a single module python library)
- Split the API reference page into "Basic" and "Advanced" for millennials lacking a 10-minute YouTube video on how to use the library
- context managers now also handle pipes' addresses, open/close status, & static payload width settings, in addition to the TX address
- automatic acknowledgment feature does not require dynamic payloads feature unless appending the ACK with a user-defined payload
- `RF24.what_happened()` now also prints RF data rate, RF power amplifier setting. The `dump_pipes` parameter now also prints the TX address, and pipe addresses have been manipulated to output exactly how the nRF24L01 uses them.
- added option to not reset the address using `RF24.close_rx_pipe()` via the `reset` parameter

To better suite the conventional CircuitPython library structure, I had to rename and move the python source around. Beneficially, the import statements are a little less wordy, but ultimately not backwards compatible with previous releases of this library. The updated import statements can be seen in the example files.

mpy files and example files are in the correct place in the zip files!
Additionally, this release is mostly improvements to the http://circuitpython-nrf24l01.rtfd.io/ and a few of the following bug fixes:

* multi-payload list passed to send() now returns a list of results per each corresponding payload.
* fixed bug in payload_length setter
* fixed typo in what_happened() output prompt

For some reason, **travis CI is not putting the rf24.mpy file in its proper place**. When you copy rf24.mpy to your circuitpython device's "lib" folder, be sure to:
1. create a folder named `circuitpython_nrf24l01` (inside the "lib" folder).
2. Then you can place the rf24.mpy along with a blank file named `__init__.py`.

The \_\_init\_\_.py is just as important as the folder you created; your code will look for that folder when importing `from circuitpython_nrf24l01.rf24 import RF24` like the examples do, and it will only find rf24.mpy if there's a blank \_\_init\_\_.py file in the same folder.

The repo is setup this way so future development can take on networking these nRF24L01 radios in a seperate module under the same folder.