Added Spongy Castle security provider

This fixes #47 

Android ships a cut-down version of Bouncy Castle. This results in
encryption/decryption errors in older Android versions when using AES-
CTR mode.

We have the three options: 
1. Switching to Spongy Castle (a renamed version of Bouncy Castle to avoid class loader conflicts) which adds about 1MB to the size of our APK. (Which is done in this PR)
2. Switch to the (included) AES_CBC_Encryptor which I have tested with Android 4.1.2
3. Switch to an encryption mode that is supported by the Bouncy Castle version included in Android which is better than CBC.