grunt: add e2e-build task (#164)

This task will build the project and run Protractor on the minified
sources. This will be useful to ensure that our builds actually work.
travis: run e2e-build task (fixes #164)
grunt: let build.dir denote full build dir path
grunt: leave .tmp directory around after build

This might come in handy if the concatenated files need inspecting.
grunt: minimize third-party Bower components (#157, #163)

This brings the number of requests down to 8 (from 16) when loading
the root page. The size went from 1.6 MB to 0.5 MB.

The page loads noticeably faster now: before, it took about 2.7
seconds on my machine, the time is now around 2.0 seconds.
grunt: minify CSS using cssmin

The number of requests to load the index page is now 7, compared to 8
before.