Use the sympy_expr from the Expression object instead of sympifying
again.
Implement the exponential Euler method
Generate nicer code for the special case of no constant coefficient
Move all state updater registration into a single place
Include exponential_euler in the standard tests
Ignore the case (e.g. 'euler' vs. 'Euler') in state updater names
Do not treat parameters as equation variables
Add some docstrings for the exponential Euler integrator and an example
for a Hodgkin-Huxley neuron. Closes #14
Merge pull request #54 from brian-team/exponential_euler

Exponential euler integration