Provided a __hash__ method for IntegerPoint.

I noticed the new runahead limiting algorithm (max active points) was not working properly for integer suites. The reason was set union was not eliminating duplicate ```IntegerPoint```, because the class did not provide (override) the ```__hash__``` method (Python data model reference says "User-defined classes have __cmp__() and __hash__() methods by default; with them, all objects compare unequal (except with themselves)".

@benfitzpatrick - please review.