Add config option to specifiy cublas library name.

Adds an optional section [cublas] to .theanorc that lets you specify a non-default name for libcublas.so.  You can also force linking against a specific version of the library with ,e.g.

[cublas]
lib = :libcublas.so.4

The config section is optional and the standard name is used by default, so this change should be invisible to anyone who doesn't explicitly change the library name.