Revert "Patch for Postgres 8.2.6 and migrations that require an implicit conversion (such as :string to :integer)"

This reverts commit fa398d8458ecb5c1730c52d747ecc764969b1cee.
allow the "USING" statement to be specified on change column calls

Conflicts:
	activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
oops, forgot to add the real assertion!
allow types to be passed in for USING casts

This allows us so abstract the migration from the type that is actually
used by Rails.  For example, ":string" may be a varchar or something,
but the framework does that translation, and the app shouldn't need to
know.

Conflicts:
	activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
ugh, forgot to add this test to cdd90f39d796986dabf1678b3277b230dbe18961
Merge pull request #15 from tenderlove/casting

Backport explicit casting