Thread independence

Hi!

I'm using your gem in a multithreaded env, and want each thread to maintain its own sampling sequence, unaffected by other threads' activity. Therefore I singletonized the `SimpleRandom` class so that it now provides a separate instance to each thread.

This patch introduces a backward incompatible API change (no `SimpleRandom.new` anymore), so we should bump the version to `1.0.0` to make it visible. Let me know what you think!