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 pandas.  The functions are written but I don't have tests yet, so I will save that for a later pull request.