Vendor iojs/doc-tool / integrate build #695

The @iojs/website team has accepted ownership of iojs/doc-tool. As I understand it, they're currently figuring out whether to run with the existing tool, or adopt an existing tool.

On our end, to enable them to move forward we need to:

• Vendor the doctool.
• Add a script to update the vendored version of doctool.
• Modify the makefile expectations such that the doctool is handed the directory full of docs files and the output directory, instead of each file individually.

The docs themselves will still live in this repo, but the tooling for building them will live in a separate repo.