Add a NUM_JOBS env var to builds

Closes #539
Use libgit2 for `git ls-files`

One forgotten spot to modify when migrating from the CLI to libgit2
auto merge of #542 : alexcrichton/cargo/issue-539, r=brson

Closes #539
auto merge of #547 : alexcrichton/cargo/git2-for-package, r=brson

One forgotten spot to modify when migrating from the CLI to libgit2
Move to rustdoc instead of ruby's middleman

* All markdown files are now rendered with `rustdoc` into HTML
* A JS syntax highlighter, prism, is included for TOML syntax highlighting.
* A new makefile target, `make doc`, will build docs into `target/doc`