gnuplot & matplotlib

[basti]

For everyone who feels limited by the sympy plotting package, I've
added gnuplot and matplotlib support. gnuplot runs pretty well, but
matplotlib gives me a hard time, since sympy and numpy aren't very
compatible.
Anyway, if you're curious, have a look at:
http://code.google.com/p/symbide/wiki/tutorial_OtherPlottingLibraries