Added logic for determining gene size.

However, I can't run any manual tests or the gff parser tests. Mitch is
working on this independently, but this code will need to be tested after
he is done with those changes.  Further, depending on how we setup
0-indexing, the tests for this code may need to change. Removed unused
tests for the gene class.
Adjusted as per Mitch's comments
Merge pull request #12 from mitchellwrosen/feature/genelengthbugfix

Added logic for determining gene size.