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.
Judging from my file system, it seems like native windows programs tend to
use %userprofile%/Documents while the Unix based ones just use
%userprofile%. So yeah, I think the latter is a good place for us (and I
prefer it, as you say).
On 29 Oct 2014 22:27, "Jan Gosmann" <notifications@github.com> wrote:

> In nengo/runcom.py:
>
> > @@ -0,0 +1,98 @@
> > +r"""This modules provides access to the nengo RC settings.
> > +
> > +Nengo RC settings will be read from the following files in the given 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.
>
> Hm ... considering that it seems to me best to follow
> git/matplotlib/ipython. Putting a config file into the Documents folder
> seems strange to me. It is not really like a Word document or PowerPoint
> presentation you open on a regular basis and work on. I wouldn't want my
> Documents folder on OS X to be cluttered with configuration files.
>
> —
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> <https://github.com/nengo/nengo/pull/376/files#r19585042>.
>