iOS 8, keyboard dodging, and (optional)UIAlertView swizzling

Changed to use more robust method of detecting screen frame when presenting. This fixes rotation errors on the new iOS 8.

Keyboard dodging added, so when a control that utilizes the keyboard is on the alert, the keyboard will not cover the alert.

UIAlertView swizzling added for implementing PXAlertView without manually changing pre-existing code. This can be enabled by defining PXALERT_SWIZZLING in UIAlertView+PXAlertViewOverride.h. (Check the comments)