Merge pull request #62 from astrofrog/plotting-improvements

Added custom_position option to DendrogramPlotter.sort
Merge pull request #63 from astrofrog/refactor-io

Refactor I/O code
Allow custom merge test function to be passed
Fixed semantic logic
Make coords= and intensity= in merge function keyword arguments, and test for orphans with custom function.
Renamed merge_test_function to is_independent
Update API for ``is_independent`` function, and added docstring
Added simple test for ``is_independent``