Add .check and .uncheck commands for immutable checkbox operations

[reallymello]

This will add the .check and .uncheck commands for changing the checked/selected state of toggleable input elements like checkboxes to checked or unchecked taking into account their current checked/selected state (e.g. .check() won't click an already checked checkbox input, .uncheck() won't click an unchecked input).

This is a useful feature so that users don't have to first check the current checked/selected state of the element before issuing the click command (or not) to alter the checked state. Additionally, its less error-prone and more easily conveys the intention versus issuing the check command.

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