Created FileHandler class

This will be used to manage file IO to allow note storage whilst the
application is not running.

Signed-off-by: Jamie Redding <jamieredding2694@gmail.com>
Created a JUnit test class and some FileHandler additions

Wrote a constructor for FileHandler to initialize fields.

Signed-off-by: Jamie Redding <jamieredding2694@gmail.com>
File writing in functional

You can now 'export' a note store to an xml file. A test class has also
been created to test the functionality.

Signed-off-by: Jamie Redding <jamieredding2694@gmail.com>