Deploy all branches but master to subdirectories

Currently using an unmerged PR in gh-pages. (Will update to an LLK fork if we decide to go this way.)

Move to Travis build stages to handle the multiple deploy methods by branch.

Stop "testing" in more Node versions than 6, as this is basically just testing our build tools (using a single Node version simplifies using build stages.)

Also add a `watch` command for testing static build output.