This commit adds integration tests against some crates in the
nursery.
Each integration test is added as a separate build-bot, where
the rust-lang-nursery/${CRATE} is first downloaded and its tests run.
Afterwards, `cargo fmt --all` is applied to the crate, and the tests
are re-run. If the tests fail after formatting, the integration test
fails.
The crates that currently fail are added as allowed-to-fail, but the
intent is that either these crates or rustfmt should be fixed such
that the tests pass.
