Regression Tests

Sequential Taggers

Add tests for:

• make sure backoff is being done correctly.
• make sure ngram taggers don't use previous sentences for context.
• make sure ngram taggers see 'beginning of the sentence' as a unique context
• make sure regexp tagger's regexps are tried in order
• train on some simple examples, & make sure that the size & the generated models are correct.
• make sure cutoff works as intended
• make sure that ngram models only exclude contexts covered by the backoff tagger if the backoff tagger gets that context correct at all locations.