RMSProp learning rule implementation

The implementation is based heavily on the AdaDelta learning rule.  Tests still needed -- should hopefully be able to get around to that over the course of the next day.  

The motivations and math behind RMSProp are described by Geoffrey Hinton at the lecture slides below: http://www.cs.toronto.edu/~tijmen/csc321/slides/lecture_slides_lec6.pdf.
A restructured text reference would be better here, so that it shows a hyperlink rather than an URL in the generated documentation.