Removed unnecessary Regex constructors for #353

As per #353, `StringHumanizeExtensions.Humanize` and `StringHumanizeExtensions.FromPascalCase` no longer call the Regex constructor on each call. `Humanize` uses the static `Regex.IsMatch` method, and `FromPascalCase` instantiates its needed Regex once during the static constructor. See the new call traces below (taken from running the unit tests.)

![image](https://cloud.githubusercontent.com/assets/1567104/5233932/6ae58c16-776e-11e4-902e-9b901f92cb50.png)
![image](https://cloud.githubusercontent.com/assets/1567104/5233943/ea66a9de-776e-11e4-8796-e705ee648536.png)

Agreed, but that option is not supported in portable class libraries, so we're stuck with interpreted expressions. This could be worked around by removing the regex object entirely and "pre-compiling" the expression to a function ourselves, but I'm not sure the added complexity would be worth the performance. Perhaps I'll give it a try, just to see what benefit there is.