[Patch] Add Factorial and cube root support to deskcalc

[jscipione]

Add support for factorials and cubed roots to ExpressionParser and thus to Deskcalc. To get a cubed root just do cbrt(expression) for example:

cbrt(27) = 3
cbrt(7) = 1.91293118277

To use the factorial functionality put a ! after the expression. Examples:

0! = 1
3! = 6
(2*4)! = 40320
exp(5)! = 2.277161E259
0.5-((0.5^3)/3!)+((0.5^5)/5!)-((0.5^7)/7!) = 0.47942553323412
pi! = 1.0876234426477

These are a couple of nice functions supported by MAPM that I wanted support for in Deskcalc.

[jscipione]

Patch that Adds factorial and cube root support to Deskcalc. Try it out, it is fun!

[stippi]

Patch looks perfect, thanks a lot!

If I don't get around to commit it soon, please someone do it for me.

[stippi]

Applied in hrev42690, thanks a bunch!