Try composer autoloading first.

Try to use composer's autoloader and if it fails, fall back to our own
autoloader. This makes it possible to use optional features (eg. logging)
when the code is run from a composer installation.
Add erebot/plop to require-dev.
Use interoperability faults if possible.

Switch to interop faults.
Some places may still need to be adapted, feel free to open an issue
if you find any.

Fixes #4.
Add tests for interop faults.
Get rid of headers processor.

We now let libxml handle the requests directly, meaning there's no more
need to mess with $http_response_header.
Also, add logging through Plop in the CLI.