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.
1. I'm ok with either keeping or removing `$NENGORC`. It seems the consensus is to remove it.
2. I would argue in favor of having a system wide config. That allows the administrator of a multi-user system to set up a specific default cache location (maybe there's some extra fast mass storage).
3. As the user might not be able to edit the system wide file, a user specific file is needed.
4. I am in favor of the precedence order. This allows to change single options without having to copy or creating the whole runcom. This allows easy sharing of projects where a specific setting might be relevant without overwriting all of the runcom of another user (though normally the runcom shouldn't include settings that important). The other reason is that if I have projects with individual runcoms and want to change something globally I do not have to edit each runcom (as long as I have not specified the setting there).
5. Not automatically installing a default configuration file makes sense. We could install an example file with some suffix like `.example` and everything commented out.

@drasmuss The linked article is about XP. [This article](http://blogs.msdn.com/b/patricka/archive/2010/03/18/where-should-i-store-my-data-and-configuration-files-if-i-target-multiple-os-versions.aspx) says the correct location for local per user configuration files is `%USERPROFILE%\AppData\Local\<MyCompany>\<MyApp>`. But we might or might not want to use [`roaming` instead of `local`](http://windows.microsoft.com/en-ca/windows-8/what-appdata-folder). I don't completely understand the implications of having it `roaming` or not.