added sequence, coding sequence, and UTR properties to Transcript objects



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`Maybe abstracting out substring methods for pyensembl`

That sounds way more terrifying to me! 

Maybe this will bite me terribly in the future, but the indexing logic seems pretty straight-forward. Ensembl is base-1, Python is base-0. When using indices from ensembl into a python string, subtract 1. Having objects with base-1 indices I think would create way more opportunities for bugs. 

Also, any idea how to unit test the indexing in a way that wouldn't be already covered by unit testing that the correct sequence gets pulled out?