MAINT: Refactor tests.

See discussion in https://github.com/biocore/bipy/issues/48
MAINT: Add `__init__` files to all tests folders for consistency.
MAINT: Update travis.yml to new package structure.
added framework for new code
added test framework
initial methods for base class
added NucleotideSequence object
added DNASequence and RNASequence classes
added properties
added basic tests
added more tests
more tests, and a bug fix
added NucelotideSequence tests
added missing tests
added more missing tests
updated for convenience of working with vim
added failing tests
improved functionality of repr

Before:
======================================================================
FAIL: complement functions as expected
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_core/test_sequence.py", line 183, in test_complement
    self.assertEqual(self.b1.complement(),DNASequence("CTAATGT"))
AssertionError: CTAATGT != CTAATGT

After:
======================================================================
FAIL: complement functions as expected
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_core/test_sequence.py", line 183, in test_complement
    self.assertEqual(self.b1.complement(),DNASequence("CTAATGT"))
AssertionError: <NucleotideSequence: CTAATGT (length: 7)> != <DNASequence:
CTAATGT (length: 7)>
fixed failing test of repr
fixed failing tests, added equality operators