new pruning. 20% speedup for basic generative parser.
improvements to span parser
Merge branch 'master' of github.com:dlwh/epic

Conflicts:
	build.sbt
	src/main/scala/epic/trees/HeadFinder.scala
Merge branch 'master' of github.com:dlwh/epic

Conflicts:
	build.sbt
	src/main/scala/epic/parser/GenerativeParser.scala
ok, refactoring actually done.