Support different user-defined actions

[jbmorley]

The actions associated with a release should be customizable at the API level to allow apps to name them appropriately. For example, it might make sense to have a 'Get' action in OpoLua, but both 'Download' and ‘Install' options in Reconnect.

[jbmorley]

This might actually make the API naming a little clearer as the callback can be something like perform action: Action.