Changed the block-scoping tests to check shadowing.

Shadowing (where an inner scope's same named variable shadows an outer-scope identifier, but doesn't replace it) is arguably a more commonplace feature of block-scoping, and the compilers tend to support it even if they don't support identifiers fully leaving scope (as in e.g. temporal dead zone). This closes #305.

The compilers had in fact already taken this into their account for their results, mostly. This corrects Traceur's result for BLFD.
Merge pull request #315 from webbedspace/let-shadow

Changed the block-scoping tests to check shadowing
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.