Allow extractor specifiable by a DSL.
Add extractors to cascades.
Use variables in extractor definitions.
Add extractors to relnoun patterns.
Fix whitespace issues.
Rename TaggerEntry to RawLevel.
Improve comments.
Cascade.apply should return extractions.
Fixup: compile error
Fixup: fix bug in ReVerb taggers.

The previous logic did not work because the token was consumed.
Preventing the token after an extraction from being a relation avoids
capturing (John, said, Paul) instead of (Paul, should, eat his spinach)
in "John said Paul should eat his spinach").
Fix bug with empty extraction field in webapp.
Merge pull request #41 from allenai/extractor

Create an extractor specification language.