Don't rebuild dependencies if they haven't changed

This commit adds support for recognizing "fingerprints" of upstream
dependencies. When a dependency's fingerprint change, it must be rebuilt.
Currently the fingerprint unconditionally includes the version of the compiler
you're using as well as a specialized version depending on the type of source
you're compiling from:

  - git sources return their fingerprint as the current SHA. This will
    disregard any local changes.
  - path sources return their fingerprint as the maximum mtime of any file found
    at the location. This is a little too coarse and may rebuild packages too
    often (due to sub-packages), but this should get the job done for now.

When executing `cargo compile`, dependencies are not rebuilt if their
fingerprint remained constant.
Merge pull request #25 from alexcrichton/fingerprints

Don't rebuild dependencies if they haven't changed