Fix Function.get_parameters() with 1 literal as its parameter

Greetings,

I started using sqlparse with a project of mine and noticed that calling get_parameters() on Function tokens with single Literals as parameters returns an empty list (i.e. "SELECT foo('somestring')" --> []).  After some headdesking, I saw that get_parameters takes into account single Identifier tokens but not Literal ones, so I added cheapo support for it and uhh voila.  Attached is a (rather shoddy) fix and the test case associated with it.  Let me know if this is too hacky.

Thanks a ton