Start rewriting tcf module not using lxml element proxies.
Add textstructure support to TCF API.
Adapt mallet exporter example to new API.
Add TCF serialization to new API and improve the API in some parts.
Port tcf.service and examples to new TCF API.
Add support for the graph layer to TCF API.
Add some graph methods to the TCF API.
Add find_top method to tagset API to find the topmost tag in a hierarchy.
Add the export method to the ExportWorker service.

The method is empty in the base class and has to be overriden in subclasses.
Add some missing pieces and fix some bugs in the new TCF API.
Implement an Edge proxy class around igraph.Edge for a more pythonic API.

The Edge class returns Vertex instances as source and target, not vertex IDs.
Add prefix support to mallet exporter.
Fix NLTK tokenizer because of API change in TextCorpus.
Add support for services passing TextCorpus objects instead of strings.
Split XML tree updating from serialization in TextCorpus.
Handle case where an ISOcat PID has no name in a MappingTag.
Remove unused DC-2446 (STTS in ISOcat) tagset.
Add serialization function to tcf.

Workers can return TextCorpus objects or byte strings (and custom workers
might also return lxml ElementTrees). Instead of checking for the type each
time, a `serialize` function has been added that turns each of the possible
returned objects into a utf8-encoded byte string.
Add support for NamedEntities in tcf.
Remove graph option 'type' in favor of automatic type assignment.