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?
Should elephant even be targeting the current ``neo`` release?  Perhaps it should be targeted at the next version of ``neo``.  There are a lot of convenience functions in the next ``neo`` for dealing with attributes and annotations that are going to be important to ``elephant`.  Further, the API changes in the next ``neo`` are going to be major and could have substantial impacts on ``elephant``, so supporting both could be a major issue.  

It is going to be a while before ``elephant`` is ready for a proper release, so I think developing it with the intention that it will only work with ``neo`` 0.4.0+ is a good goal.