Add documentation to Configuration class

Document valid keyword arguments and point to them from op2.init.
Use builtin logger everywhere

Previously we were sometimes using warnings.warn, switch to using
logger.warning for those cases.
Write dumping of generated code into all backends

Rather than being opencl-specific, make it so that the dump_gencode
parameter causes generated code to be output on all backends.
Additionally, switch to using the md5sum of the generated code as the
uniquifier, rather than the time the code was generated.
Remove unnecessary global statement