Add the packaging metadata to build the pyethapp 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 that then can be distributed to many users through the Ubuntu store.

[come-maiz]

Hello!

Here at ubuntu we are working in a new packaging and delivery system. If you haven't heard about snaps before, here's a lot of info: https://snapcraft.io/

Just as I mentioned in the similar PR I opened for serpent, I'm personally very interested in making more blockchain software available to the users of Ubuntu. We have millions of users, but installing these new projects is hard for them because they are not in the debian archive. With snaps, they will be easily available even for users in old stable releases, and even for users in other distros that support snaps.

To give this a try in an Ubuntu 16.04 machine:

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

Please, let me know what do you think.

[come-maiz]

Thanks for the merge @janx! Once you publish it in the store, we can help organizing the ubuntu community to test it.

[janx]

@ElOpio thank you!