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.
Add word sense discrimination layer to TextCorpus.
Add support for references and named entities.

The previously unfinished support for named entities was completed and support
for references added.
Add support for references and wordsenses.
Use given POStag instead of generic one as node type.
Add a proxy class TokenList that connects tokens and reference/entity elements.
Add layers to corpus using a method instead of attribute setting.
Remove deprecated `find_token()` method.