chore: move sorting methods in prototype to the constructor

Useful to use them when we there is no gridster instance, like sorting
a previously stored output from the serialize method.
style: some formatting changes
feat(gridster): move widget up when added if there is space available
fix(gridster): sort widgets appropriately when reading them from DOM
feat: make gridster AMD compatible
feature(gridster): added `auto_init` config option, default to true

Useful when using `positionschanged` event, fired before gridster
instance is cached.