Add RC file support

This pull request adds support to configure the cache size and path with RC files. Settings will be loaded from the following files (in this order):

1. `INSTALL/nengo-data/nengorc`  (where INSTALL is the installation directory
    of the Nengo package)
2. A operating system specific file in the user's home directory.
   - Linux: `~/.config/nengo/nengorc`
   - OS X: `~/Library/Application Support/nengo/nengorc`
   - Windows 7: `%userprofile%\AppData\Local\CNRGlab at UWaterloo\nengo`
3. File given in the environment variable ``$NENGORC``.
4. nengorc in the current directory.

See `nengo-data/nengorc` for an example RC file.

This pull request addresses ctn-waterloo/nengo#349. It is based on the `cache` branch. Thus pull request #375 should be merged before this one. This pull request also changes the cache limit to disk space instead of number of files.
If we wanted to make this stupidly simple, we could leave the defaults where they are now and just have one optional config file in the user's home directory. No project-specific stuff. It's less flexible, but for the kind of stuff that we're doing it should work, and it makes things very easy.

If we want to keep going in the direction of this PR, I would take all the defaults out of `runcom.py` and just have them in `INSTALL/nengo-data/nengorc`.