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
Rename runcom to rc

Before this branch, I didn't even know that RC stood for runcom.
Actually, I just googled it, and it's apparently short for
"run commands." In any case, this is one of the acronyms that
used to have a meaning but is now mostly known just by the acronym,
so I think we should just embrace it and use `rc`.