bugfix, consistency fix

This patch makes rendering of the differential or partial differential operator in the different printing systems consistent, addressing issue 1339. the latex renderer always used the partial symbol, while the mathml and unicode renderers always used a plain d. now some attempt is made to check whether the derivative is of a multivariate function or not.

also, there was a bug in the rendering of derivatives in latex where higher order derivatives were rendered as d^n/d^n x instead of d^n/dx^n.

the method for determining whether to apply the partial or full derivative could probably be made more sophisticated and accurate, but I view this as an improvement over what exists.

Also, I have developed a set of unit tests for the diffgeom package checking the output for the hyperbolic half-plane against manual calculations. This test also serves as an example of using diffgeom (hopefully of use to others).

https://code.google.com/p/sympy/issues/detail?id=1339