[Fixes #429] FileResult

1) Implemented FilePathResult to efficiently return files from disk.
2) Implemented FileStreamResult to return content from a stream.
3) Implemented FileContentResult to return content from a byte array.
I don't see a strong reason to derive from FileResult. Should FileResult turn into a helper class? There is not a use of FileResult as an id anywere, it's just a convenience baseclass, hence it can probably go away.