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.
We should perhaps get some feedback from other people on this. It's difficult to find the right balance between ease-of-use for novice/non-technical users and flexibility for expert users. I feel that the axis/NaN approach adds too much complexity (dealing with NaN's in particular is tricky), but on the other hand the `axis` argument is optional and can be ignored by novices.