added generic generator code; updated univariate and discrete
bugfix draw-bernoulli, final fixes for adding generators

added random-number-generator.lisp: base class, cl wrapper, and gsll
added GSLL dependency for call GSL's random number generators
added draw-bernoulli-bit: same as draw-bernoulli, but returns 0/1 instead of NIL/T
bugfix draw-bernoulli: return boolean for integer probabilities instead of 0,1
added tests for bugfix