Added onBackPressed Event for Creating Keys.

Average time required to create a key is 3 minutes. If the user accidentally presses back key. He has to go through the entire process again. So added a new alert dialog. 
I have also changed the button text to "Change Passphrase" while editing keys if passphrase is set.
 
and 


#207 Issue has been fixed.

https://drive.google.com/file/d/0B31e8A3Srhc3TGRwSExrYU1rSzg/edit?usp=sharing

3rd Commit: I have looked at Issue 307 . I changed the implementation (onBackPressed()) to Dialog Fragments.

4th Commit: #379