Import/export from/to YAML file

[corot]

Until we don't have an editor, this will be the canonical way of creating a semantic map and saving on database. In fact, in many cases this is more practical than a graphical tool, as we learned while preparing the cafe concert demo.

[corot]

Done. YAML format is a list of pairs annotation + data messages, so N-1 is not supported, nor it is arrangement by world id or others.
That means that the current simplistic format will change in the near future (see issue #9)