Issue #166 Oracle does not support locks in subquery

Since Oracle doesn't allow you to lock a row in a subquery, this will get the row and lock it. I ran the tests against Oracle, MySQL, and SQLite.

There is still one broken test on Oracle that was present before this change. Line 767 of awesome_nested_set_spec.rb fails because oracle won't let you use a clob in a where clause (what it translates column type :text to). I suspect that is an issue that needs to be fixed in the adapter.