More functions and cleanups for elephant

This pull request does 4 main things:

1. It implements the discussed improvements in the `isi` function.
2. It adds a `firing_rate` function.  Its purpose should be self-explanatory.
3. It adds a `conversion` module, which is intended for various functions that convert between different representations.  The only function right now is `binarize`, which was discussed previously.
4. It moves the `cv` function into its own `compat` module, where we can put other NeuroTools-related compatibility functions.

All of these, including API and names (and whether we should do 4 at all) are open to discussion.

I am also working on a `pandas` interface for `elephant`, with conversion between `neo` data types and `pandas` ones, as well as helper functions for using `elephant` functions with these and other `pandas` objects.  The functions are written but I don't have unit tests yet, so I will save that for a later pull request.
A good reason to not support it is if it would confuse users, which I think is the case here. If you have, say, a 2D array, then all the rows and columns must obviously have the same length. It is essentially never the case that *n* spike trains will all contain the same number of spikes (I suppose you could imagine an analysis that takes the first ten spikes from each channel, but I've never heard of such a thing).