Update and enable embind tests to run with the Closure Compiler

This change adds a pass to the embind tests to run them with the Closure
Compiler enabled.

Because embind.test.js will be passed through the Closure Compiler, name-
mangling will occur on unquoted references to embound classes and functions.
However, because embound symbols are exposed via string literals, this results
in a mismatch.

The tests have now been updated so that all references to embound symbols are
referred to by string literals (i.e. object["property"] syntax) instead of just
by name (i.e. object.property syntax).

A set of Closure externs for Underscore were also added, because Underscore
(which is used by the embind test adapter) exposes some symbols by string
literal as well.
Update embind to work with Closure Compiler

Several names within the embind JavaScript itself need to be guarded from name
mangling in the Closure Compiler.

With this commit, the embind tests will now pass in the Closure Compiler test
phase.