list `clean` command in help
auto merge of #169 : cburgdorf/cargo/name_clean, r=alexcrichton

There are other unnamed commands like `verify-project` but I'm unsure if they are **not** listed on purpose because they should be used directly by the user less frequently. However, the clean command seems like a command that should be listed here.

Not sure about the wording though. It could be more abstract like *remove build artifacts* but given it's current behavior I think *remove target directory* is somehow straight forward, too.
Add support for examples
Add support for external tests
Do not run examples during 'cargo test'
Use PackageId in the DependencyQueue

This allows the dependency queue to properly handle packages with the same
name but from different sources.

A test was added which exercieses this functionality by depending on two
different revs of the same git repo.
auto merge of #163 : o11c/cargo/examples, r=wycats

This add support for `examples/*.rs` being built (as normal bin crates) during `cargo test`,
and `src/test.rs` and `tests/*.rs` being built and run (as test crates) during `cargo test`.
auto merge of #170 : alexcrichton/cargo/same-name, r=wycats

This allows the dependency queue to properly handle packages with the same
name but from different sources.

A test was added which exercieses this functionality by depending on two
different revs of the same git repo.