Pyensembl

EnsemblRelease` objects are memoized upon deserialization to avoid creating multiple database connections.

Very small change: increased the maximum allowed Ensembl release version to 83

- PyEnsembl is now compatible with both Python2 and Python3
- Gene, Transcript, Exon objects are all now hashable and can be tested for equality
- removed redundant location methods from EnsemblRelease
- added distance methods to Locus and search module for locating nearest locus to an interval
- fixed GTF parsing for Ensembl release 78