Explore supporting CI [appyeyor/travis]

[robertmaynard]

It would be nice to support pushing builds to CI infrastructure. This would allow users to setup workers that are actually just CI builds.

The json setup would look like:

{
"hostname" : "bigboard",
"type" : "travis",
"cpp_compiler" : "g++6",
"build_generator" : "Ninja",
"build_flags" : "-j8"
}

Notice this has a different syntax compared to the default worker. We should flag the legacy workers with a 'type' == bare_metal.