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



<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/hammerlab/pyensembl/19)
<!-- Reviewable:end -->

It'd be good to distinguish (in naming, comments, or otherwise) functionality that is only used for testing vs. functionality that's going to be (but is not yet) a part of the API vs. functionality that's a part of the API. That would help make it more obvious (to someone like me) what needs more/less testing, etc.