Expose Async.protectedCont and WebResponse.Url

I need this for the HTML Extensions, I hope it's fine.
Cleanup of Enumerator components

Moved enumerator specific conversion functions to own files.
DateTime initial implementation
Almost complete implementation of DateTime and TimeSpan

Unit Tests have also been included, and they are all working (except for
the string conversion methods, as the format may differ slightly between
Runtimes)
Fix DateTime.AddYears with edge cases like Feb 29th in leap years
Additional methods for Regular Expressions
Discard ToString() implementation in DateTime and Regex
Fix issue #111
First implementation of Timer object

No unit tests yet and it only can be used as IObservable
Fix Seq.cast
Added requestAnimationFrame helper to Core.Time
More formatting capabilities for String.Format

New Formats accepted: F, P, E and Dates (D,T,d,t). Unit tests need to be
implemented.
Fix issue #112: arguments added to keywords
Merge pull request #110 from alfonsogarciacaro/master

Implementation of DateTime, additional Regex methods and more