Better include method for iOSPort headers.

[UPDATED]   It's better to just include the iOSPort headers as groups in the project than to mess with project include header search path build settings.
PearlOverlay + PearlAlert/PearlSheet updates.

[ADDED]     PearlOverlay: A convenient way of showing an overlay on the window in an app.
[ADDED]     Ability to show a message in the activity PearlAlert.
[ADDED]     Ability to query alert/sheet visibility directly on PearlAlert/PearlSheet.
[UPDATED]   dismissAlert -> cancelAlert and make sure the alert is dismissed with the cancel button.
[UPDATED]   tappedButtonBlock is now a writable property.
Merge branch 'master' of github.com:Lyndir/Pearl