Add layers

[marcverhagen]

With the new coref code this is now becoming more and more useful.

For backward compatibility we will not change the main layout of the Tarsqi XML document, that is, we will still have the source_tags and tarsqi_tags tags, with the same content as before. But in addition we will have a layers tag with all other layers (Stanford, HeidelTime, coref, etcetera). Tarsqi components can import tags from those layers if they choose.