YAML format output for API

[maul-esel]

Additionally to JSON and XML, YAML might be a good output format to add to the API.

However, this requires the YAML PHP package to be installed, which is not shipped with PHP by default.

[maul-esel]

Previously discussed on Library-Distribution/libba.net#12.

[maul-esel]

#35 takes care of this. It also degrades gracefully if yaml is not installed.