import private keys from Bitcoin Wallet backup files

Adds a new button to the AddRecordActivity which allows the import of private keys from backup files created by the Schildbach "Bitcoin Wallet" app. Didn't create a new Activity but simply added AlertDialogs to AddRecordActivity (don't know if you'd prefer to have this in separate Activity).

Some details:
- button is not shown if no backup files exist
- works with testnet and prodnet
- allows to choose which file to import
- allows to choose which key from the file to import
- english and german strings included
- uses javax.crypto.Cipher class to decrypt the AES-256-CBC enciphered backups (uses no code from the Schildbach Bitcoin Wallet)

![device-2014-05-08-223900](https://cloud.githubusercontent.com/assets/285900/2921372/9f08505e-d6f2-11e3-87a4-e01a3e90e5d0.jpg)
![device-2014-05-08-223912](https://cloud.githubusercontent.com/assets/285900/2921374/a5f66cde-d6f2-11e3-84f4-ab78f24577a9.jpg)
![device-2014-05-08-223950](https://cloud.githubusercontent.com/assets/285900/2921378/ad2142fe-d6f2-11e3-8e25-5d4a3adb5e3b.jpg)
![device-2014-05-08-224006](https://cloud.githubusercontent.com/assets/285900/2921381/b0b63b68-d6f2-11e3-852c-4559e13c38fb.jpg)
![device-2014-05-08-224040](https://cloud.githubusercontent.com/assets/285900/2921385/b29b5710-d6f2-11e3-9ea6-ab5932efa449.jpg)
![device-2014-05-08-224119](https://cloud.githubusercontent.com/assets/285900/2921386/b50088f4-d6f2-11e3-86db-5561b565991f.jpg)
![device-2014-05-08-224131](https://cloud.githubusercontent.com/assets/285900/2921387/b742bf1a-d6f2-11e3-8c0e-2616ae1140b1.jpg)