Translated some leftover Dutch comments
Added support for NOT operator.

- added SpanQueryNot which produces all tokens that do not occur
in its clause's hits
- complex fields now store their token length, needed by SpanQueryNot
- added rewrite() method to TextPattern, currently only eliminates
double-NOTs. Gets called automatically from Searcher.createSpanQuery.
- updated CorpusQL parser for NOT functionality. Removed sequence-level
NOT operator (doesn't exist in CQL).