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.
Dotfiles like the user-level one that this branch introduces are so common in Unix that most people have a way to manage them pretty easily (e.g. [my dotfiles](https://github.com/tbekolay/dotfiles)). We're doing better than most projects by storing it a place that's totally non-intrusive (and matches the XDG base directory specification), so I don't think there's a compelling reason to get rid of the user-level `nengorc`. On Unix anyway -- on Windows it is kind of annoying, but again, not Nengo-specifically annoying ;)

There is a compelling reason to discourage users editing `INSTALL/nengo-data/nengorc`: when they do a `pip install --upgrade nengo` they'll lose their customizations. If they use multiple computers, they're also unlikely to remember that they customized something in a `site-packages` directory (which should feel super weird anyway), whereas most will try to mirror their dotfiles across different machines. Given this, we probably shouldn't tell users to edit this file -- we probably shouldn't even list it as something that we support them editing. They just need to know where it is so that they can copy it locally to edit it.