added a visitor for Set objects
Wrap group_by sql with a subquery for limiting rows in Oracle.

Fixes #128
Merge pull request #315 from tamird/129

Wrap group_by sql with a subquery for limiting rows in Oracle.
Build on ruby-head
Remove unused @alias, being referenced in hashing.
Merge pull request #316 from vipulnsward/remove-alias

Remove unused @alias, being referenced in hashing.
Informix versions < 10 use 'FIRST' keyword instead of 'LIMIT'
Still supported in versions 10+
Merge pull request #137 from dimko/visit_set

Added a visitor for Set objects
Merge pull request #167 from pparidans/informix-v9-support

Informix versions < 10 use 'FIRST' keyword instead of 'LIMIT'
Fix set visiting spec
Merge pull request #317 from dimko/patch-2

Fix set visiting spec
connections should cache these values, so remove the cache in arel
move the dispatch table to be per-instance

visitors are not shared among threads, so any mutations to the cache
should be OK.  The cache is also pre-populated on construction, but we
should pull that out so we can share the cache among visitors in the
future.
{Matches,DoesNotMatch} support the ESCAPE clause
Merge pull request #318 from tamird/match-escape

{Matches,DoesNotMatch} support the ESCAPE clause
Delete should respect 'limit'

Conflicts:
	lib/arel/visitors/to_sql.rb