Update docs and tests for equations
Add some examples for NeuronGroup namespaces to the user docs and update docstrings
Add a note about doctests in documentation to the testing guidelines
Return arbitrary values (instead of dictionaries) as a result of a
codeobject. For thresholds (the only code that has an output currently),
this means to return the spikes array directly instead of wrapping it in
a dictionary.
Add some missing newlines to the equations documentation.
Move specifiers module from codegen to core
Merge pull request #9 from brian-team/equations_rewrite

Equations/namespace rewrite
Merge remote-tracking branch 'origin/master' into stateupdater_choice

Conflicts:
	brian2/equations/equations.py
	brian2/groups/neurongroup.py