Implement NoisyCurrentSource and other sources of injecting current

[andrewgait]

A request has been made by a collab user to try to implement the various ways of injecting current on sPyNNaker (for a list of what PyNN can do, see http://neuralensemble.org/docs/PyNN/injecting_current.html?highlight=currentsource). In particular they are interested in the NoisyCurrentSource; but it may be worthwhile implementing all the possible methods.