Adding ability to monitor file changes for PhysicalFileSystem

1. This includes the implementation & tests.
2. A modified IFileInfo interface (This will require changes to MVC repo - A PR will be sent out after this changes are approved.).
3. File system watcher turns off when there are no files to monitor and turns on when files are to be monitored.
4. Need to add a test to verify that file system watcher turns off if no files to monitor.

@Tratcher @pranavkm 
This should probably only be done if your path has any wildcards. For a path that doesn't, it seems like overkill to have it run through a compiled `Regex` expression. Additionally, I would rather not use compiled expressions here - unless we know for certain that the filters are going to be used repeatedly, it might be detrimental to have it be compiled.