Support for NOT EXISTS and EXISTS.
Change Minus operator to Negate, to distinguish from Minus query operator
Add dataset minus operator to implement the SPARQL 1.1 Minus pattern algebra.
Implement SPARQL 1.1 Negation parsing.
Update travis to load JRuby 1.7.4, rather than 1.7.3.
Add equivalent-xml to Gemfile, as it was removed from gemspec.