Bug fixes and refactoring

Bugfixes:

• This crate did not accept messages that had an empty signature header field. Those are legal but carry the same meaning as if the header field was missing entirely.

Important changes:

• push_param{2-5} now fail "atomically". If any of the push calls fails, the message body is reset to the state before the pushes started
• make SignatureWrapper more generic over any AsRef and add some TryFrom impls for SignatureWrapper and ObjectPath
• bring ObjectPath UnixFd and SignatureWrapper into their own module under wire and rexport them
• refactoring: separate traits from their impls and the impls into two categories: base and container (this shouldn't have any effect on users, it's just a nice to have)

Other stuff

• The documentation has been cleaned up, updated, and fixed (in hopefully all places)
• The rename to README.md should hopefully result in a rendering of the readme on crates.io