Return Recurly::Error for all internal errors

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
No default #to_param

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Errors should behave more like ActiveModel::Errors

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Test nested models

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Make it easier to get an add on's details
Convert resource links from procs to serializable hash

https://github.com/recurly/recurly-app/pull/1440#issuecomment-18028526

Procs can't be properly marshalled, so hrefs are lost when
Recurly::Resource objects are cached. By storing links as a hash and
resolving the href without a proc, we can properly marshal them from
cache.

Acked-by: Stephen Celis <stephencelis@gmail.com>
Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Bump interal gem for specs
drop support for 1.8, add support for 2.0
Merge pull request #121 from recurly/release_internal

Release internal fixes