otr_keystore secure sync via SD Card

This is the full sync method based on putting an AES-encrypted file on the SD card.  Currently, the complete sync works, but there is no UI for handling the password or bad passwords.  To make it work right now, you need to hard-code the password in Gibberbot.

The other side is fully implemented in [otrfileconverter](https://github.com/guardianproject/otrfileconverter).  The workflow is this: 

1. attach your phone and mount the SD card
1. choose the phone's SD card as the output folder
1. launch `otrfileconverter-gui` or run `otrfileconverter` from the terminal, it will generate a file called **otr_keystore.ofcaes** in the output folder, then display a QRCode of the password for Gibberbot to scan
1. unmount the SD card and launch Gibberbot
1. Gibberbot will detect the **otr_keystore.ofcaes** and launch the password-scanning GUI
1. after a successful scan, it will import the contents of **otr_keystore.ofcaes** and delete the file from the SDcard