Pass through extra arguments to `cargo test`

This allows `cargo test` usage to filter test being run, use --nocapture, etc.
auto merge of #161 : alexcrichton/cargo/cargo-test, r=wycats

This allows `cargo test` usage to filter test being run, use --nocapture, etc.
Pass --cfg ndebug when debug = false

This will disable debug!() and debug_assert!() statements.
auto merge of #180 : sfackler/cargo/ndebug, r=wycats

This will disable debug!() and debug_assert!() statements.
Add documentation and make minor variable renames