Rio-tiler

------------------
- Fixes sentinel-2 band 8A regex bug in `rio_tiler.utils.expression`

------------------
- adds DatasetReader input option for utils.tile_read (do not close the dataset on each read)

Breaking Changes:
- `utils.tile_band_worker` renamed to `utils.tile_read`
- `main.tile` **rgb** option renamed **indexes**
- `sentinel2.tile`, `landsat8.tile`, `cbers.tile` **rgb** option renamed **bands**
- `main.tile` default nodata mask is handled by rasterio

------------------
- adds utils.b64_encode_img function to encode an image object into a base64 string
- add tiles profiles (jpeg, png, webp) based on https://github.com/mapnik/mapnik/wiki/Image-IO#default-output-details

Breaking Changes:
- Refactor `rio_tiler.utils.array_to_img` to return PIL image object

------------------
- only using `read_masks` for mask creation when it's needed.

------------------
- add "expression" utility function
- better nodata/mask/alpha band definition and support

Breaking Changes:
- tile functions now return an associated mask (Landsat, Sentinel, CBERS, main)
- remove nodata support in utils.image_to_array function
- add mask support in utils.image_to_array function
- utils.tile_band_worker will always return a (Band, Width, Height) array (e.g 1x256x256 or 3x256x256)

------------------
- remove aws.py sub-module (replaced by main.py)
- no default bands value for main.py tiles.