Adding capabilities to specify a canonical transcript list (from a text files) and fixing a bug with caching for EnsemblTranscriptProvider.
Adding unit test for "-" in TCGA MAF and fixing bug with custom canonical tx lists.
nice little syntax cleanup
added test for empty canonical transcript list
Merge pull request #273 from broadinstitute/issue-72-Custom_canonical_list

Provides the ability to specify a custom file with specific transcripts to prefer
closes #72 custom canonical list
closes #229 when a list of transcripts matching the uniprot proteins is provided  ```-c test/testdata/tx_exact_uniprot_matches.txt```

also adds a test case to cover the case of a failure raised in #271