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.
How would it be confusing?  I think the use of the `axis` argument is pretty ubiquitous in python data libraries.  I am not sure how anyone could have used `numpy` (or matlab, for that matter) for more than a few minutes and not have encountered the `axis` argument.

As for the sort of analysis that could lead to that, I think it is best to keep things flexible.  It is up to the users to decide what sort of analyses they want to do.