Improve callbacks by passing the view as a parameter

This way one can directly call the `fadeMeOut` method on the actual view instead of having to call `[TSMessage dismissActiveNotification]` (which would be necessary if you want to dismiss the view programmatically in the callback, because you have disabled manual dismiss via `canBeDismissedByUser:NO`).

I find it better to deal with the actual instance of the view rather than relying on it being the currently active notification.

Right now it is only possible for it being the currently active notification, but these changes will make sense in conjunction with the next changes I'd like to propose in the next commits. ;)
Add permanent messages

Permanent notifications differ from normal ones in that permanent messages are not contained in the messages queue and can be displayed in addition to the other messages.

Permanent notifications do not get dismissed automatically, hence they do not have a duration but have to be dismissed by the user or programmatically in one of the callbacks.
The constructor for a permanent message also returns the instance of the message view, so that one can also capture it and dismiss it with other custom events.
Add permanent messages to demo
Do not automatically fade out on button tap
Remove iOS 6 compatibility and clean up

This is so we can go forward and remove some of the incomplete edge case handling when calculating height and offset of the message view in `fadeInNotification:`, which depends on the statusbar and navigationbar being present or not.

The demo project now works for all cases and shows the buttons to toggle statusbar and navigationbar.
Remove iOS 7 style remainder
Fix check for image from custom design file
Refactor initializers
Rename showOrEnqueueNotification method
Move custom design handling to TSMessage class
Remove xccheckout file from repository
Remove state variable
Use dispatch_once for singleton initialization
Remove unused defines
Consolidate naming

notification -> message
show -> display
fadeOut -> dismiss
Update README
Improve interface with convenience methods
Fix naming I oversaw
Merge branch 'develop' of https://github.com/dennisreimann/TSMessages into develop