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.
this link is no longer relevant if it's not using Plovr. https://github.com/google/closure-compiler/wiki/FAQ#when-using-advanced-optimizations-closure-compiler-adds-new-variables-to-the-global-scope-how-do-i-make-sure-my-variables-dont-collide-with-other-scripts-on-the-page (yes, the url really is that long) describes the advantage of having a wrapper with advanced, but the problem doesn't really exist with the other two modes as it doesn't create extra globals.