Brian 1 Hears bridge

The majority of the work for the bridge between Brian 1 hears and Brian 2 is now complete. Most examples run with very little change, see ``dev/ideas/b1h_bridge``.

Still to do (but maybe not necessary for now):
* ``Sound.duration`` returns a Brian 1 unit, and similarly for all properties
* Usage examples in the ``examples`` directory? Maybe not?
* Tests?
* Documentation? e.g. explaining what changes are necessary.

Fixes #232.