Validate some simple generators early

This makes the crashing easier to diagnose, as it will indicated an
invalid generator was passed in. It's not 100% ideal, but most
generators are complex and simple static validation is not enough. We
can do a lot better but would probably require quite a bit more work
Merge pull request #118 from basho/bugfix/validate-early

Crash if uniform_bin MaxSize is not greater than MinSize