The extension static media packaging code attempts to install

dependencies needed for the pipelining process. If *.less files are

provided, it will install LessCSS. If *.js files are provided, it will

install UglifyJS.
This solves one of the more annoying issues with extension packaging,

which is getting the right versions of the right dependencies going.
Subclasses can also augment the installation behavior by overriding

install_pipeline_deps().

Tested building an extension without having lessc or uglifyjs in

my path, and with a clean source tree. Verified the generated output

in the packages.
Tested with and without having npm in the path. Saw an error when it

wasn't found.
Tested the case of npm install ... failing.