Merge pull request #71 from astrofrog/fix-example

Fix example to follow latest analysis API
Merge pull request #75 from astrofrog/coveralls-io

Added coverage testing via coveralls.io
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``
Use benchmark examples to further test is_independent
Added section to docs about using ``is_independent``