remove the models/ sublibrary

It got extracted as metaruby and is unmaintained
remove a pile of warnings issued by ruby's -w option
fix error raised by WeakRef when the argument is frozen

define_finalizer is now (on ruby 2.1) raising RuntimeError
(instead of ArgumentError) in this case. Catch and re-raise
ArgumentEror
test: port to minitest

Ruby's bundled minitest version and the gem are out of sync (in general),
which makes Test::Unit break. Simply port to minitest, and while I am
at it use a common template for test setup (from Rock's ruby library
template)
test: fix backtrace filtering test

The test block moved, so we need to update the lines
use singleton_class? to provide is_singleton? for ruby >= 2.1