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.
I checked, and I think it's more common to put these files just in `%userprofile%` in Windows.  That's where ipython, matplotlib, and git all put their config files.  It would be a lot more convenient to work with than the current location, and as @tbekolay points out we don't have to worry about cluttering up people's `%userprofile%` because it's optional, so it'll only be there if they want to put it there.