Parallelizable Locate

Refactor locate to make it easy to run in IPython.parallel. It relies on the ``frame_no`` property added in soft-matter/pims/#29 to add a frame column, so frames so do need to be analyzed in order.

See [this example notebook](nbviewer.ipython.org/github/danielballan/trackpy/blob/parallel/examples/parallel-locate.ipynb). I haven't linked that to the README yet -- I'm thinking about a "Cookbook" page in the docs instead, though self-contained notebook are nice....