Support CircleCI for building

CircleCI is an awesome alternative to travis for CI. https://circleci.com

Using a simple .circleci/config.yml (below) you can have circleci use an Ubuntu 16.04 image to build using snapcraft, however we don't currently have support for adding the store credentials in the same way we do for travis.

CircleCi supports using encrypted credentials. I documented how I got that working at https://forum.snapcraft.io/t/building-pushing-snaps-using-circleci

https://github.com/circleci/encrypted-files

version: 2
jobs:
  build:
    working_directory: ~/emoji
    machine: true
    steps:
     - checkout
     - run:
         command: |
           sudo apt update
           sudo apt install -y snapd
           sudo snap install snapcraft --edge --classic
           /snap/bin/snapcraft
     - deploy:
         name: Push Master to Snap Store
         command: |
           openssl aes-256-cbc -d -in .snapcraft/snapcraft.encrypted -out .snapcraft/snapcraft.cfg -k $KEY
           /snap/bin/snapcraft push *.snap