Remove unused import

Related to #100.
sample IDs are now just ids

Also removed DistanceMatrix.num_samples as this is just len(dm.ids) or dm.shape[0] or dm.shape[1]. Seemed like an unnecessary method...
add support for construction from a DistanceMatrix
__getitem__ supports ID pair lookup
Fix pep8 error
add numpydoc docstrings to DistanceMatrix
add numpy docstrings for module and sym dm
remove extra whitespace
add missing word to module docstring
update some dm docstrings
remove raw python docstrings except where necessary
Add exceptions to module docstring
random_distance_matrix -> randdm

Renamed function to be more pythonic. Updated constructor and random_fn parameters to have default values of None, as the documentation that was generated was pretty horrible looking.
minor docstring updates; thanks @gregcaporaso!
Merge pull request #112 from jrrideout/issue-109

DistanceMatrix improvements and numpy docstrings