Provide a way to specify which JVM browsersim is using

Default would be the default jvm (the one running with eclipse)
but should be able to point to another jvm/execution environment.

One gotcha is that to run we need bit-matching version of swt which means if you use a 32 bit jvm on a 64 bit eclipse (or vice versa) we need to have the "opposite" swt jars available.