Experiment saving status to cookies
Experiment saving game state to localStorage

The localStorage mechanism is not supported by Internet Explorer under 7,
but we have already using HTML5 canvas, which is not supported by those
older browsers.
Also, localStorage have an advantage in that it can be used by "file"
protocol with the local html file.
Pack state information of towers into a JSON string

We can serialize and deserialize it to save/load state easily.
Add check of previous save data existence prior to loading