Implement a web backend for rustdoc_ng

This large commit implements and `html` output option for rustdoc_ng. The
executable has been altered to be invoked as "rustdoc_ng html <crate>" and
it will dump everything into the local "doc" directory. JSON can still be
generated by changing 'html' to 'json'.

This also fixes a number of bugs in rustdoc_ng relating to comment stripping,
along with some other various issues that I found along the way.

The `make doc` command has been altered to generate the new documentation into
the `doc/ng/$(CRATE)` directories.

Previews
* http://www.contrib.andrew.cmu.edu/~acrichto/doc/std/
* http://www.contrib.andrew.cmu.edu/~acrichto/doc/extra/

Missing features
* Different versions of documentation on the same page (all possibly indexed as well?) I think that this needs to be thought out before action is taken. It's an awesome idea, but it should be done carefully.
* Source links are missing. This is a little dependent on getting versions working. In theory we should link back to github, but we should always link back to the exact version the documentation was generated from.
* Integration with other tools. It would be awesome to have rustpkg-style inference of the package name and version so they don't have to be specified anywhere. Additionally, I should be able to build documentation for a pkgid, not necessarily a crate file.

cc @cmr/@Seldaek