Depreciate 'target' parameter on handlebars action helper

The 'target' parameter is legacy in actions of the form:

{{action "methodName" target="App.mainController"}}

Full paths can be used as follows:

{{action "App.mainController.methodName"}}

Apart from following a more intuitive and natural flow, as the full handlebars hash (key/value from the tag) is supplied to the callback the target param tends to 'pollute' the call; we should depreciate the use of 'target' as it is now unnecessary. Although the target parameter still works, I have added a depreciation warning if it is used, with the intention of removing it circa v0.2.0.