Fix the README
GitSource delegates to PathSource for path ops
Allow absent git version
Only force update git if -u is passed

This will become moot once we have a lockfile, because we'll be able to
check whether the last rev we used is still available and skip fetching.
Rename cargo-compile to cargo-build
Work around \-characters on Windows for now
Merge pull request #31 from alexcrichton/hotfix-windows2

Work around \-characters on Windows for now
Update README.md
Clarify `make install` exists as a target

Closes #39