Data File Encryption

I've tackled issue [#66](https://github.com/nurupo/ProjectTox-Qt-GUI/issues/66).

These commits add a class *Profile*, responsible for saving/loading/encrypting a given Tox *Messenger*. The file format used is described [here](https://github.com/jencka/ProjectTox-Qt-GUI/blob/data-saving/src/profile.hpp#L16) (it's loosely based on Toxic's design). I've also created a [quick-and-dirty branch](https://github.com/jencka/ProjectTox-Qt-GUI/tree/data-saving-test) that shows *Profile* in use.