Random time value

Currently this is still a very half-assed setup but I'm opening a PR for some early feedback. The reason for this PR is the following issue: https://github.com/rubinius/rubinius/issues/2412

Current TODO:

- [ ] Support Date instances
- [ ] Support DateTime instances
- [ ] Make the setup a bit more generic (e.g. call `#to_if`/`#to_i` on the min/max if possible)

p.s. I wrote this while half asleep, just a fair warning ahead.