pg guide, add index to PostgreSQL array example. [ci skip]
rm cached attributes

The original patch that added this concept can be found
[here](https://web.archive.org/web/20090601022739/http://dev.rubyonrails.org/ticket/9767).
The current default behavior is to cache everything except serialized
columns, unless the user specified otherwise. If anyone were to specify
otherwise, many types would actually be completely broken. Still, the
method is left in place with a deprecation warning in case anyone is
actually still calling this method.
Basic attribute object
Make column and serialized type serializable
Use type_cast_for_database
new, dup, serialize working
WIP
WIP
WIP
PASSING TESTS ON SQLITE3
Remove uses of type_cast_for_write
Passing tests on mysql
Do or do not.
PASSING TESTS
Bytea need not know about binary
Don't rely on the changes to `[]`