Optimize exon count accumulation a bit
Add comments and remove unused code
Validate strand in exon index
Make some changes to strand support

In ExonIndex, use 1 to mean +, - to mean minus, 0 to mean
stranded but unknown, and None to mean no strand.

Validate strand when adding to the index or querying it.
Support unknown strand
Add some logging
Merge branch 'develop' of github.com:itmat/pre-pade into develop