`Token` should not be able to have whitespace

[schmmd]

Unfortunately, BreezeSentencer uses Tokenizer.computeOffsets to compute offsets from the resulting sentences, so simply adding require(string.forall(!_.isWhitespace)) breaks BreezeSentencer.

[schmmd]

@rbart, @jgilme1 postag and chunk no longer can contain whitespace FYI. b4fdb99