Changed get_min|max to min|max_key
Updated docs and made index.cursor function.
Merge branch 'master' into api-update
Fixed Python2/3 bug in gtf2wt and tests.
Fixed get_max bug on index and test cases.

This required a change in the index file format where we now append a
zero after every column, not just the internal ones.
Removed extra parameter.
Minor updates to tests.
Made start/stop systematic accross tools and tests.
Updated tutorials and fixed minor bug.
Updated examples to new API.
Added discussions of half-precision floats.
Changed cache_size -> db_cache_size + documented.
Added make entry to create htmlzip.
Merge branch 'api-update'