Place SaveFile writer in using block

The "finally" block in SaveFile() does not check to see if the
streamwriter is null before attempting to close it. If the writer has
not been instantiated for whatever reason, such as an
AccessDeniedException, the method throws an unhandled
NullReferenceException. Placing the writer in a "using" block will
make the code cleaner and automatically handle the aforementioned case.
Merge pull request #59 from tsliang/master

Place SaveFile writer in using block