Change order of test and trial functions

FEniCS/DOLFIN has used a "non-standard" order of test and trial functions for a long time (at least since 2005) and for good reasons. However, as we get more users a large percentage of those are faced with an unfamiliar way of expressing variational forms. Most literature also uses the a(u, v) notation. For reference, see this discussion:

http://<email address hidden>/msg00174.html

Changing back to u, v will require (small) changes to UFL, FFC, UFC and DOLFIN.