DOC - fix docs in tracking.utils to reflect that the affine argument is not optional
DOC - fix intro section of dipy.tracking.utils for better web rendering
DOC - move most of tracking._utils to tracking.utils so that the docs
will show up on the website
BF - add back future import
Small clean up for PR
Merge pull request #352 from MrBago/fix_utils_docs_and_affine

Fix utils docs and affine