[#3] Fixed placement between the alert and the keyboard

Changes made regarding an issue #3:
- added class `OLGhostAlertWindow` which is responsible for displaying
an OLGhostAlertView above the application keyWindow. It has methods to
show/hide alert above the application main window, which allows it to
be independent of the system keyboard.
- removed properties in `OLGhostAlertView` that are no longer needed:
`keyboardIsVisible`, `keyboardHeight`
- added new properties in `OLGhostAlertView`: `alertWindow`,
`viewController`
- modified methods responsible for initialization, showing/hiding the
alert and laying out subviews
- removed methods and notifications related to the keyboard, which are
no longer needed: `keyboardWillShow:`, `keyboardWillHide`,
`UIKeyboardWillShowNotification`, `UIKeyboardWillHideNotification`
- set method `showInView:` as deprecated