Integrate clippy into the build system and Continuous Integration (CI)

[briansmith]

#277 shows that Clippy is useful. We should integrate it into the build system and/or the continuous integration so that we can catch problems before they get committed.

On Continue Integration, Clippy checks should run AFTER the tests. I think it would be too annoying to have a push fail to even run the tests because of a formatting error.

It's probably best to use the cargo clippy subcommand instead of the compiler plugin, as I suspect it would be less brittle and more likely to work across stable/beta/nightly Rust.

[briansmith]

See https://github.com/Manishearth/rust-clippy/issues/902 regarding the bug where Clippy warns about backticks for path components of URLs.