Adds heart.system namespace functions, corresponding to love.system

This changeset implements all but `love.system.getProcessorCount()`, because that particular function can't be done cleanly in JS without pulling in an external library https://github.com/oftn/core-estimator . Whether pulling in that library to enable one, likely rarely-used LÖVE function is worth it is up for debate.

All the functions work to some extent in all the browsers, but due to security restrictions a lot of it does not work as nicely as one would wish. Also, the HTML5 Battery Status API is not available in browsers other than Firefox yet, and there is no other way to access that information; fortunately that API will soon be implemented in Chrome (it's already in WebKit itself).