HAL::Links module shouldn't prepend an underscore to links

According to the docs the links key should not have an underscore.
That should be added when the HAL module in mixed in and overrides
the links_definition_options.

https://github.com/apotonick/roar/blob/master/lib/roar/representer/json/hal.rb#L84
require ostruct, our new best friend.
make HAL::Links work as it is documented, namely just like HAL::JSON but with "links" key. thanks @csexton for fixing.  https://github.com/apotonick/roar/pull/56
simplify HAL::links_definition_options as it relies on its super method in HAL::Links.
releasing 0.11.8.