Custom method + on/off custom values

[cristianciofu]

Gives the user the ability to :

• add a custom made function with whatever parameters he may need
• add custom values to on/off values for the cases where the model is not based on true/false values

[cristianciofu]

Thanks to @andrejkaurin (https://github.com/andrejkaurin)

This change was made after seeing his comment here : https://github.com/xpepermint/angular-ui-switch/issues/21#issuecomment-147652780

[cristianciofu]

@xpepermint any updates regarding this PR ?