[Import] Implement and test references

[MartinKl]

The Import of co-reference annotation is neither implemented nor tested so far.

[MartinKl]

WebAnno seems not to use the attributes ID and TARGET in tag REFERENCE. It groups the references by their parent tag ENTITY and chooses the first or the last token (by id) as target in the visualization.

[MartinKl]

Right now ignoreIDs only works with an antecedent in the last position.