implemented term negation
fixed string comparison bug on ruby 1.8.7
Added a tsearch-specific :negation option to enable term negation
Merge pull request #118 from derek-watson/master

Implemented simple negated terms

Conflicts:
	lib/pg_search/features/tsearch.rb
	spec/lib/pg_search/features/tsearch_spec.rb
Add test for when options[:negation] is false

#118
Add integration test coverage for negation

re: #118
Add documentation for :negation

re: #118
Merge branch 'negation'

Closes #118