Enable closure compiler ADVANCED_OPTIMIZATIONS in the build script

[mrjatkin]

Requires externs files to be written / generated for the third party libraries we're using

[mrjatkin]

We need an externs file to enable advanced javascript optimisations. Even though I've ripped out a couple of third party libraries, I really don't want to write the externs files by hand, because it would still take ages to do. As a result, I plan to create a tool that generates the externs file for us. This will be developed as a separate project and will be open sourced, as it would be useful to other people too.