Add support for Authy App Tokens (e.g. Twitch.tv)

Fixes incorrect decryption.

Due to incorrect unpadding for some tokens where the secret length
was not aligned with the AES block size (16 bytes).

BUG: Truncation of CBC padding was wrong

Resulting in wrong decryption for large secrets. Whoops.

Update README with importing hint

Add licence and try fix build

Initial commit