Add Firefox add-on SDK to build dir

Add Firefox add-on SDK v1.16 submodule.
This provides the cfx tool that will be used to create
and test the Firefox add-on as part of the Travis CI build.

In future we'll be able to use cfx' successor jpm tool
installed via npm. That currently appears to require
Firefox >= 33 however so we'll stick with cfx to support
the current stable version of Firefox
Setup deployment of Firefox addon to S3 as part of Travis CI build

Build the Firefox add-on using cfx and deploy to S3
as part of Travis CI builds of the master branch