Plovr-free build

This PR is not ready to be merged, but I'm opening it for discussion.

This PR adds code to build ol3 without Plovr. If you want to try this, you'll need to:

Checkout the latest version of the compiler source into `build` (needed for contributed externs):

`$ git clone https://code.google.com/p/closure-compiler/ build/closure-compiler`

Checkout the latest version of the library source into `build`:

`$ git clone https://code.google.com/p/closure-library/ build/closure-library`

Get the latest compiler JAR file and build generated sources:

`$ ./build.py build/compiler.jar build/src/external/src/exports.js build/src/external/src/types.js`

Now you can build ol3 without Plovr:

`$ python bin/pyplovr.py bulid buildcfg/ol-plovr-free.json > ol.js`

Notes:
 * `bin/pyplovr.py` uses the same JSON build config files as Plovr
 * a few source modifications were needed to build with the latest version of the library
 * the generated code is slightly larger, I don't currently know why