add a choice of slide backend: openslide and tiffslide

[kaczmarj]

replace openslide with tiffslide https://github.com/bayer-science-for-a-better-life/tiffslide

tiffslide is implemented in pure python and should make distribution easier.

[kaczmarj]

let's wait to incorporate tiffslide until Bayer-Group/tiffslide#72 is resolved. i have found that tiffslide is slower than openslide when reading patches from a TCGA BRCA slide.

[kaczmarj]

instead of removing openslide, we have included a choice of backend, tiffslide or openslide in #139

this will require uses to install openslide or tiffslide separately. let's think about how to do this... what should be listed as a required dependency in setup.py?