Removed ES6 compiler test pages from version tracking.

Since these are not public-facing, take a good while to re-compile, and make more complicated diffs, there's little reason to keep them tracked.
Fixed build.js to create the compilers directory.

Also, it will no longer build the compiler pages without the new "compilers" command-line argument.
Merge pull request #339 from webbedspace/compilers

Removed ES6 compiler test pages from version tracking