Make Worker chainable. !!Contains API break!!

The Worker class is now chainable. One can build chains programmatically by
using this syntax:

    'A string' | AWorker() | AnotherWorker()

This changes the Worker API: Now, the input_data parameter is not passed to
the contructor, but instead to the run() method.
Add RemoteWorker that allows to call TCF web services.
Add example tokenizer based on NLTK.
Improve list splitting in tokenizer example.

Now the list splitting function discards empty elements at the end.
Always output XML declaration (for WebLicht compatibility).
Rewrite the nltk tokenizer example to use more of the TCF API.

The tokenizer is now a better example of the TCF API. It uses more of the API
sugar and works less directly on the XML level. However, this comes at the
cost of speed. It makes the example a good showcase (which is its point), but
for production use, a faster implementation, either of the tokenizer or of the
API, will be advisable.
Change tokenizer from NLTK's default to WordPunctTokenizer.
Add remote service example.
Add mallet exporter example.