Start writing functions

The aim of this pull request is to start some discussion.

1. opinions on docstring formats?
2. data formats - is everyone happy to use a Quantity array to contain the inter-spike intervals, for example?
3. warnings and NaN - coefficient of variation of inter-spike interval needs at least two spikes. If there is a single spike in the train, I have chosen to return NaN, but should we raise an Exception, or at least a warning?
I think as a general policy we should avoid combining functions like this is a good idea.  First, I just think in general that separation of concerns is a good policy.  More practically, if someone wants the ``cv`` but also wants to use the ``isi`` for other things, it would require calculating the ``isi`` twice (or more if we do this in many functions), which can be a big performance bottleneck on large data sets.  And for people who just the cv, ``cv(isi(spiketrain))`` is only one more character than ``cv_isi(spiketrain)``.