This fixes segmentation fault in the tests as extending the examples in a before block with override of `#instance_eval` seems to not work.
Also, the context wrap is now opt-in with a helper method, instead of opt-out and relying on `#described_class` (which has slightly different behavior in RSpec 3).
Tested on Ruby 2.1.5 and RSpec 2.99.2.
This changes the accessor code to push as much
object allocation onto Ruby as possible. Instead
the C++ layer acts strictly as glue using stack
allocated variables to convert v8 value to ruby
objects at the moment it passes them off to Ruby.
