Support empty prompts, have the prompt cell take up no space when the prompt
is an empty string.
Use a tiny, but non-zero, layout size to avoid log warnings. Make prompt
changes work after the component is already visible. Add prompt testing
capabilities to the test app; now when you type in the UIResponder test
text box, it also sets the picker prompt.
Change to reload only the prompt cell when changing the prompt; causes proper
display with no risk to other selected cells. Also use copy semantics when
setting prompt property, as declared in the .h file.
Merge remote-tracking branch 'upstream/master' into emptyPrompt, and update
the sample view to work with the changes.

Conflicts:
	Sample/MBContactPicker/Base.lproj/Main.storyboard
	Sample/MBContactPicker/ViewController.m
Move prompt setting text field to top of interface so that the keyboard never
gets in its way. Also add a switch to test/demonstrate the new property which
controls whether already-selected contacts appear in the list of completions.
Merge pull request #37 from brunchboy/emptyPrompt

Support empty prompts, have the prompt cell take up no space when...