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?
On the question of releases, it seems to me that Neo 0.4.0 is quite a long way away, whereas an Elephant 0.1.0 release could be done quite soon. (Note: I just saw that the name "elephant" has been claimed on PyPI, although there is no package there).