DistanceMatrix -> DissimilarityMatrix, SymmetricDistanceMatrix -> DistanceMatrix

Still need to update unit test code.
fix doctest failing test
update distance module unit tests

Still need to update the rest of skbio to use the new class names.
update rest of codebase to use new names
Fix some minor issues with the docs
Adding percent explained property to PCoA
Fixing PCoA bug on eigvals returned
update "Gain in PD" reference
Adding test case for eigvals bug
Adding ids to the PCoA results
Addressing comments
Solving merge conflicts
Returning also 0.0 eigvals
MAINT: Update required numpy version
TST: Also test numpy 1.7
MAINT: Remove unused import.
TST: Replace self.assertTrue(np.array_equal... with npt.assert_array_equal.

The former wouldn't work in numpy <=1.7 (before this fix https://github.com/numpy/numpy/commit/6dc709a5aa0b17b5a74b7456d7cbd2580d9063f6#diff-297f089c1998527812c2b0d9975afa0d)
MAIN: Remove unused import
TST: Same thing in PERMANOVA as in...
... dac1bcfad0f0483f9f5725bb5b8938bfaafdc28
Merge pull request #252 from Jorge-C/1.7

np 1.7