~~~~~~~~~~~~~~~~~~~~~~~~~~
- rewrote `ensembl.get_protein_coding_genes()` function to use pandas for
parsing data, store more information about the genes (including position and
orientation), and add gene sorting feature
- added `ensembl.gdc` subpackage, which includes routines for querying the
RESTful API of the NCI's Genomic Data Commons (https:///gdc.cancer.gov)
- minor fixes