Implemented comparison on Locus objects

[iskandr]

'gene.exonsdoes not work on Py3 (https://github.com/openvax/pyensembl/issues/215) due to lack of comparison forsorted`, fixing this by implementing basic comparison methods on Locus (base class for Exon).

[julia326]

Travis failure:

FAIL: test_transcript_start_codon : Check that fields Transcript
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment-/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/build/openvax/pyensembl/test/test_transcript_objects.py", line 31, in test_transcript_start_codon
    CTNNBIP1_004_locus, Locus.__str__(CTNNBIP1_004_transcript))
AssertionError: Expected locus Locus(contig=1, start=9850659, end=9878176, strand=-) but got Locus(contig=1, start=9850659, end=9878176, strand=-)

[julia326]

(See Travis failure)

[coveralls]

Coverage decreased (-0.4%) to 79.936% when pulling f11718b on comparison-on-Locus into 6329a03 on master.

[iskandr]

Standardized magic methods (including __str__) for Exon, Locus, Transcript, Gene objects, added unit tests for __str__.

[iskandr]

@julia326 OK to merge?