Implement basic support for encrypted text files

You may want to store secret or private information on your mobile
phone not intended for other eyes; this patch implements basic support
for reading and modifying encrypted text files.

For de- and encryption the "Gibberish AES" library is used
(https://github.com/mdp/gibberish-aes), mainly because of it's size and
speed. The library will decrypt files encrypted with AES-256-CBC, thus
making it compatible with OpenSSL (and several other libraries).