Hdf schene representation, manipulation and saving

This is going to be a long pull request with a lot of functionality. first of all, there are some fixes and minor features which I should have contributed as a separate PR, but the fact that I'm the only contributor so far justifies some slacking, I hope.

The big stuff: I rearranged a lot of the functionality so that it works not on an in-memory list of trajectories, but on HDF5 files, which can be memory-mapped so take up no memory and are additionally faster. The HDF5 trajectory format already exists in flowtracks, and it has now been made central. Other formats are easily convertible to it using ``flowtracks.io`` to read, then write back.

A tutorial exposing the new workflow, main classes and features has been added to the new ``doc/`` subdirectory. It is an IPython notebook, which you can view here:
http://nbviewer.ipython.org/github/yosefm/postptv/blob/hdf_schene/doc/hdf5_scene_analysis.ipynb