Implementation of DateTime, additional Regex methods and more

Hi Zach,

As the title says, I implemented almost complete versions of DateTime and TimeSpan with working unit tests.

I also cleaned up a bit my previous implementation of Enumerators for imperative collections (ResizeArray, Dictionary, MatchCollection...), added more expression replacements to the Regex module and exposed a couple of things which I need for the HTML extensions: Async.protectedCont and WebRequest.Url. I hope this is fine. 

I added a line to CommonOperators.fs to fix issue #111 too.

Please have a look when you have time and tell me what you think.

Cheers!
Alfonso