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.
Alright, I've been reading up on Microsoft specs for this kind of thing, and I'm pretty sure the appdirs answer is wrong (at least in our case).  Here's the MSDN article appdirs references re: where to put user data http://support2.microsoft.com/default.aspx?scid=kb;en-us;310294#XSLTH3194121123120121120120.

But that article says
>Applications work with two types of documents: those that the user creates and those that the application creates...

The former they suggest should be in `My Documents/appauthor/appname`.  It's only the latter that should be in `AppDirs/appauthor/appname`.  It seems like our user-created config files would fall into the former camp.  git/matplotlib/ipython seem to have split the difference and put the config files in the common parent of `My Documents` and `AppDirs`, which is `%userprofile%`.

What has my life become :weary: