Several improvements

Built upon sgryschuk/SGNavigationProgress#11

- Fixed bug with changing the title back when presenting a view controller directly after loading is completed.
- Allows multiple loading title changes to indicate several loading stages. (e.g. Authenticating, Retrieving data)
- Added methods for setting a percentage animated with a specific duration. Could be used to load from the network. For example at the start of a request you can animate to 90 % in 3 seconds and when the data is actually loaded you can finish with the `finishSGProgress`
- Sets the `tintColor` to the same value as the `navigationBar.tintColor`.
- Animates more smoothly by using `UIViewAnimationOptionBeginFromCurrentState`