Support for clearing Script Cache

[AndersMad]

Thanks for the contrib - I'm using this as a replacement as the prior use of the vanilla lib for the nice support of caching scripts etc.

I see that the project uses LruCache instead of System.Runtime.Caching, but the later got support for HostFileChangeMonitor/FileChangeMonitor - which could be nice to auto dispose the cached entry on the basis of the script(s) files changes. But since file change monitor is currently not an option - I'd like to suggest a method for clearing the cache manually; Would be much appreciated :) (its currently internal/private)..

E.g. (a thread safe) cal: ClearScript.Manager.ManagerScope().ClearCache()

[eswann]

OK, will take a look when I get a few...feel free to add it and do a pull request! :)