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.
Definitely don't get rid of the user-level file. If Nengo is installed system-wide (e.g. to `/usr`) and the current user doesn't have root priviledges, then they need a user-level file to set configs across all their projects. Even if they do have root, I prefer putting all my config stuff in my home dir.