Add in_realtime method

[stevebrewer]

Easy way to execute some code in the real time without having to call Timecop.reset and then re-freezing / traveling when you're done. My particular use case was testing posting to S3, which will verify the timestamp that's part of the signature it's too far off from realtime.

[travisjeffery]

Hey, I thought about it and decided to modify Timecop#return to temporarily return for the given block if given one, otherwise if not given a block it behaves as before and returns for all subsequent lines. I'd like to improve the documentation of everything either tonight or tomorrow night and then I'll ship the next gem version. Thanks!