[vcs] A minimal vcs module.

This supports checking whether a directory is a vcs-controlled directory
as well as generating a version number for a vcs-controlled directory.
[vcs] Expanded available operations.

- fetch
- checkout
- root-url
- ref
[package-manager] Added a preliminary package manager.

This is a preliminary package manager that aids in downloading packages
from within Boost.Build.  This still needs a lot of work, but is good
enough to show how it might work.