Added variable PBKDF2 rounds

PBKDF2 rounds are calculated at Vault creation and stored in Vault
header. It currently targets about 1 second for the key creation. Target
and minimal allowed rounds can be set in the Config class.
Alert user of restore overwrite
Delete old vault (preset parent of first file) before restoring
Include backup finish text to short notif text
Merge pull request #41 from L-Henke/variablePBKDF2Iterations

Added variable PBKDF2 rounds
New CipherInputStream for EXTREMELY improved decryption performance

The original Java CipherInputStream uses a 512 Byte buffer which is
extremely slow. Since that code is licensed under the GPL, I could not
simply modify that file.

I now have modified a CipherInputStream from Bouncy Castle which is MIT
licensed.

The decryption performances is now exactly as the encryption
performance. The performance improvement compared with the totally
unoptimized version is about factor 10!
Renamed values in Config class
Merge pull request #42 from Doplgangr/backupImprovement

Perfect!
Fix import screen FC
Fix (dirtily) leave vault bug
Fix BackupDoneEvent not handled,
Since some unknown fragments also gets this event (don't know exact reason behind) without initialized mBuilder.
Add not null checker.
Merge pull request #43 from Doplgangr/exceptions

Exceptions
Merge pull request #44 from L-Henke/SecrecyCipherInputStream

New CipherInputStream for EXTREMELY improved decryption performance