Remove Plovr

This PR removes Plovr and relies on closure-util and the build.js and serve.js scripts instead.

This PR adds a jquery-1.7.js externs file, which is used for the compilation of the examples. This file is modified (0ae1cd7) to fix an issue in the official externs file (maintained by the closure-compiler folks). A pull-request to the closure-compiler has been [created](https://github.com/google/closure-compiler/pull/523).

Please review.

fixes #2222.
Currently, as I see it, our "main" script for building is still build.py. That is why I changed `npm start` to `build.py serve` here.