Support defined? with ::constant (fixes #319)

This ruby syntax now returns nil for undefined constants, or 'constant'
if it exists. This commit adds 2 specs for this syntax.
Support regexp options in %r{} notation (fixes #318)

This commit allows regexp literals constructed using %r{} notation to
also accept regexp options/flags. Specs also added.
Fix buggy heredoc parsing (fixes #317)

This commit adds support for wrapping heredoc wrapper in quotes. This
allows custom characters to be placed in heredoc markers.
Fix NilClass#to_n to return null
Fix masgn with empty splat (fixes #316)

Empty splats are now just ignored as they dont have any useful value
Add ruby 2.0.0 back to travis, but allow failures

For some reason (our fault? rubys fault?) 2.0.0 will sometimes fail.
This commit adds 2.0.0 support to travis so the tests will still be run
against ruby 2.0, but we can observe the results without causing our
builds to fail everywhere.
Add asset builders to Builder (compatible with sprockets)

Allows sprockets style requires and handlers, without the sprockets.