Make EntryElement.BecomeFirstResponder(bool) virtual so it can be overridden

We have a MultilineEntryElement which requires this change so it can inherit from EntryElement.

We want it to inherit from EntryElement so the loop in EntryElement:263 will consider MultilineEntryElement's to become first responder.


On a side note, we have some other local changes you may be interested in, I can send PRs for any you'd like :) (They were mostly implemented by @jacobtoye)
- Added an Ended event to EntryElement which is fired when it stops being first responder. (We use it to save the field to the DB)
- MultilineEntryElement - uses a UITextView to be a multiline EntryElement, with placeholder text:

![Screen Shot 2013-02-10 at 4 02 09 PM](https://f.cloud.github.com/assets/393086/142858/5bab125c-732e-11e2-8691-ce7b0ea16881.png)