Add the packaging metadata to build the tx snap

[come-maiz]

This is a package for the secure installation of apps that works in most Linux distributions.
Landing it upstream will enable builds for adventurous users.

[come-maiz]

You can find more information about snapcraft here: http://snapcraft.io/

To test it in an Ubuntu 16.04 machine:

$ sudo apt install git snapcraft
$ git clone https://github.com/elopio/transifex-client
$ cd transifex-client
$ git checkout snapcraft
$ snapcraft
$ sudo snap install *.snap --dangerous

If you have any questions or comments, please let me know.

[come-maiz]

ping. Anyone around to review this?