Fix recursive mutex locking issue on JRuby when using Sequel::Model(dataset) (Fixes #841)

There's already a model spec for this, which does fail when run
in isolation.  For some reason it passes when you run the model
specs and the core specs together (which the default spec task
does).
Use all_tab_cols instead of user_tab_cols for defaults parsing in oracle adapter

user_tab_cols only shows tables the user owns, while all_tab_cols
shows all tables the user has access to.
Use all_tables and all_views for Database#tables and #views on Oracle

This works better for users with limited rights.  Similar to other
databases, we try to hide the system tables.  In the case of Oracle,
that's not all that simple to do, as there are a lot of system users,
so we may want to expand the IGNORE_OWNERS constant in the future.
Make rake default task run plugin specs as well as core/model specs

People that don't read the testing guide or look at the output of
rake --tasks might not know that you need to run spec_plugin for
the extension and plugin specs.  This should make it more likely
that new users actually test extension/plugin changes before
submitting pull requests for them.
Update rdoc for PGRange#empty?