use correct parameter names for gamma distribution functions

Incanter used the 'rate' as parameter name, although in reality this was
a 'scale' parameter (https://en.wikipedia.org/wiki/Gamma_distribution).
For pdf-gamma, cdf-gamma & sample-gamma functions the new parameter
':scale' was introduced (equivalent to the old ':rate' parameter), and
':rate' parameter is now the '1/:scale'...

Also, the arguments of Probability/gamma wasn't used in correct order...

This fixed #245