Add continuous integration with CircleCI

[adrianhall]

We want to do automated deployments to npmjs.org - using CircleCI for the process.

[adrianhall]

Added files and created release branch. Needs set up on CircleCI side and testing.

[adrianhall]

We are now building the release branch to be auto-released.

New process for release:

npm version {major|minor|patch}
git push
git checkout release && git merge master && git push
git checkout master

Whenever we push to the release branch, the CircleCI will be run.