Make union/except/intersect chainable as SelectManager

Fixes rails/arel#118
Fixes rails/arel#98

Conflicts:
	lib/arel/nodes/and.rb
	lib/arel/nodes/select_statement.rb
	lib/arel/visitors/mssql.rb
	lib/arel/visitors/to_sql.rb

Conflicts:
	lib/arel/select_manager.rb
Update manifest
Fix breaking test