Added keyboard shortcut to toggle between active layouts (Issue #148)

I have added the functionality to save user's last 4 input method choices in browser's local storage.
We can toggle between these 4 choices and the default System Input method using the keyboard shortcut (Ctrl + L).
It works well on Firefox and Chrome.
I tried to test it on IE under virtualbox but the keyboard shortcuts were not working altogether, not even the disable IME shortcut Ctrl + M (Is it a bug?).

Local storage is supported across all major browsers. See http://caniuse.com/namevalue-storage
The total number of choices to be saved, and the keyboard shortcut key to toggle input methods can easily be changed if required.