Allow separate setup and build frontend tasks

[gapple]

More expensive operations like npm install && bower install only need to be run on initial setup and when dependencies change.

Separating them into a frontend:setup task would allow frontend:build to run fewer operations and take less time during development.

[sarahjean]

++

[sarahjean]

Would also be good to document caching the npm and bower assets in .travis.yml

[grasmash]

@gapple @sarahjean If I run npm install or bower install against a directory that already has built dependencies, won't it just skip installing them?

[sarahjean]

@grasmash That'd be nice but it doesn't seem to behave that way. I think composer is a little better behaved in that regard.