Move Random methods out of Stage class

[shallawa]

https://github.com/WebKit/MotionMark/issues/54

This is a step towards making Stage and its sub-classes be ES6 classes. To do this the Random methods will be moved out of Stage class.

A new class Random will be added to hold the random methods as static methods. This class will be put in a separate file. A production script can be used later to merge all the JS files in a single file.

[smfr]

This needs rebasing.