Converted Chronic to a portable library

I've converted Chronic to a portable library.

To make it compile I had to remove the RegexOptions.Compiled flag used when creating regular expressions as it is not supported in SL and WP.

I also changed Math.Truncate with Math.Floor in two places as Math.Truncate is missing from SL and WP. Floor and Truncate behave in the same way for positive numbers, which was the case in those two places.

I want to use the library in a SL project, so that's why I did this. If you accept the pull request, and push it to nuget, I will get it from there. Otherwise I will just use the binaries compiled from my branch.

Regards,
Eugen