Small doc update.
General reshuffling around of files for better packaging.
Further folder rearrangement.
Make header file more readable.
Move C++ extension to subpackage.

This creates room for a thin Python layer that we can use to make
calls to the distance functions more flexible.
Fix Travis configuration.
Initial code for handling tau=0 as a special case.
Add test for tau=0 and improve test for small tau.
Update docs; add proper docstrings to functions exposed to Python.
Dev version bump.