Makes client, common, response, and request modules public; fixes build.

Fixing the build consisted of:
    1. Adding new rust-url dependency.
    2. s/std::sync::atomics/std::sync::atomic/g
    3. s/std::ascii::StrAscii/std::ascii::Ascii/g
    4. Fixing up ownership problems in examples/websockets.rs
        (This ended up being a larger refactor of the example)