Implement the gpg functionality

[dashohoxha]

Complete the application by implementing the gpg functionality on the prototype.

Subtasks:

• Research and test the gnupg related libraries. Decide which ones to use.
• Get familiar with GPGME and try out its examples.
• Implement the code for managing the key.
• Implement the code for managing the contacts.
• Implement the code for encrypting/decrypting files etc.

Prerequisites: #7
Related tasks: #2