Add Support for 2/4 bpp Tiff Grayscale Images

[zwhfly]

Including inverted and/or bit-order-reversed formats.

Taking 2bpp non-inverted reversed format as example, the term 'reversed' is interpreted as follows:
8 bits of a byte: MSB---> ABCDEFGH <---LSB
unpacks to:

pixel 0 value: MSB---> HGHGHGHG <---LSB
pixel 1 value: MSB---> FEFEFEFE <---LSB
pixel 2 value: MSB---> DCDCDCDC <---LSB
pixel 3 value: MSB---> BABABABA <---LSB

This interpretation is the same as the existing image processing tools that I've seen (ImageMagick and Windows Photo Viewer).

Sample pictures can be found here: Tests/images/tiff_gray_2_4_bpp/hopper*.tif

[coveralls]

Coverage increased (+0.08%) to 76.958% when pulling 124bee0 on zwhfly:tiff into 4d234fb on python-pillow:master.