Remove DateTime.Now

This PR fixes the `DateTime.Now` garbage generation in the TouchPanel that @Nezz found in #2948.

I also went ahead and purged all uses of `DateTime.Now` in the runtime code replacing it with `DateTime.UtcNow`.  Not only is `DateTime.UtcNow` garbage free, but it is [several orders of magnitude faster](http://blogs.msdn.com/b/kirillosenkov/archive/2012/01/10/datetime-utcnow-is-generally-preferable-to-datetime-now.aspx).