Added configparser to compat

The capitalization changed in Python 3.
A few edits to nengorc documentation

Took a few hints from matplotlibrc.
Nengo RC file size now given with units

The user specifies something like "512 MB" rather than an
integer that is assumed to be in bytes.
To do this, `bytes2human` and `human2bytes` functions were added.
Cache get_size returns a human readable str by default now;
get bytes by passing `in_bytes=True`.
Cache testing improvements

- Fixed test_cache_performance, which wasn't updated to newer syntax
- Added test_cache_works as a minimal smoke test similar to
  test_cache_performance, but much faster
Remove prompt numbers from benchmark notebook
Fix imports in nengo.cache

Some weren't used; nengo.runcom was unnecessarily long.
Changed get_default_dir to staticmethod
Removed network_seed_set

It wasn't being used inside `get_default_decoder_cache`.
No reason why it shouldn't use the decoder cache if the random
seed randomly matches, am I right?
Removed Epydoc-style variable docstrings

I don't think we'll ever generate Epydoc for Nengo.
Shortened runcom import in conftest.py
Add appdirs license to LICENSE.rst