add `builder` config option

Allows user to specify a custom builder for zuul other than the default
browserify builder.

The builder should be a commonjs module that exports a single function:
function(files, cb). Files is an array of files and once the builder has
built them, it should call the callback with (err, src, map) where map
is an optional sourcemap object.

close #41
update to localtunnel 1.1.0
1.6.0
rename qunit.js to framework.js

npm seems to hate symlinks
1.6.1
phantomjs run fails if nothing passes
pass options to bundler builder
1.6.2
Retrying to instantiate a browser when getting error status 13 from wd
Retrying to intialize the browser when a non JSON response is received with data indicating that the run is not in progress
Added a retry threshold of three (3) to retrying on errors indicating that environments can't be instantiated so that we won't end up looping forever by mistake.
Merge pull request #50 from rase-/error-workaround

Retrying to instantiate a browser when getting error status 13 from wd
1.6.3