Pil-lite

- build extension with CFFI
- drop CPP extension code, rewrite it on C
- drop cpython extension code
- drop requirement to have python shared library
- support pypy
- drop py2 support
- py3.5+ type annotations, mypy checks
- basic drawing functionality: create empty image, get/put pixel, draw rectangle
- numpy array support: np.array(img) and img.fromarray(arr)

- update stb_image (v2.22), stb_image_write (v1.13) and stb_image_resize (v0.96) libraries
- bump python version to 3.7, set travis tests to 2.7 and 3.7 only
- added Image.show function (linux 'display' only right now)
- bmp output
- replace nosetests with pytest
- limit input formats to png, jpg and bmp
- limit outpug formats to png, jpg and bmp

- update stb_image, stb_image_write and stb_image_resize libraries
- use jpeg write functionality from stb_image_write
- remove jpeg-compressor dependency
- bump python version to 3.6 (latest tested)

- fix GCC 5 compilation (temporary)

- fix error message if image fails

- remove boost-python dependency