Reference

Created TranscriptReferences object for downloading and accessing FASTA reference file for transcript IDs. Also, moved GTF dataframe methods from EnsemblRelease to new GTF object. 
Not quite the same:
```
>>> "a.gtf.gz".split(".gtf")
['a', '.gz']

>>> os.path.splitext("a.gtf.gz")
('a.gtf', '.gz')
```