Add the packaging metadata to build the pocketplace snap

[come-maiz]

This package will let you publish the latest pocketplace in the Ubuntu store, and from there reach many users on all the supported Ubuntu versions, and more Linux distributions in progress.

[come-maiz]

Hello!

Here at Ubuntu we are writing a new packaging and delivery system. You can read more about it here: https://snapcraft.io/

We are working on a new release, and used your application for some tests. It would be nice if you take a look, let us know what do you think, and if you like it, go to https://build.snapcraft.io/ to publish it in the store.

To test this in an Ubuntu 16.04 machine:

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

(--dangerous because the snap you build doesn't come signed from the store. Once you push it, all your users will just have to run $sudo snap install pocketplace).