[ui.mask]: Adding new option ui-mask-raw to enable/disable binding the raw or view value to the masked input model.

Following the implementation proposed by @Snede in #199 i improved the implementation of this extremely needed feature.
I added a global configuration in the ui.mask module called uiMaskRaw (as suggested by @ProLoser).
If it is set to true (default) the behavior is the same that we have today, the raw value is assigned to the model.
If it is switched off (false) the view value (masked value) is assigned to the model.

I also observes the ui-mask-raw attribute to override this configuration locally, as follows:
<input type='text' ui-mask='(999) 999-9999' ui-mask-raw='false' />

That way you can set the global behavior and the individual input behavior.