Adding logic for mouse following focus.

When focus changes, SIWindow queries the Configuration to see if the user would
like for their mouse cursor to follow the focus change.  If so, it will position
the cursor in the midpoint of the window.  The default setting is for this not
to happen.

Also, tidied up the logic for ignoreMenuBar to be consistent with the rest of
the codebase.
Cleaning up mouse follows focus logic.

Instead of extending the SIWindow class with a method of the same name,
I added a new method `AMFocusWindow` which AMWindowManager now calls.
This method calls the original `focusWindow` method of SIWindow
and then performs mouse follows focus if it is configured.  Also added
a brief README entry for the setting.
Code Cleanup

* Rename AMFocusWindow to am_focusWindow to follow conventions.
* Proper conditional when checking the return value of focusWindow in
	am_focusWindow
* Inline midpoint
Merge pull request #113 from leonardt/mouse-follows-focus

Adding logic for mouse following focus.