Tweak the two Cargo.toml files

* The docopt library doesn't need to specify that it is built as a dylib, that's
  generally an end-user decision which cargo will allow.
* Use a path dependency instead of a git dependency so docopt_macros always uses
  the local version of docopt when building
Merge pull request #16 from alexcrichton/toml-changes

Tweak the two Cargo.toml files