Recover from various issues

Closes #33

We have problems when documents are not in sync regarding their token structure. This can happen even with valid documents, such as when Arethusa inserts elliptic nodes.

There were also errors when partially unannotated documents were supplied (missing attributes, or in the case of the postag also an empty string). This is also taken care of here.

While these minor changes keep the application running and not throwing exceptions, there are some follow-up problems with it. A new issue will deal with them.