Rebase/update 2-0-stable branch

[sohara]

The 2-0-stable branch appears to be missing some crucial commits from master such as ad66845. Since there's probably nothing API breaking in master and several things that fix other breakages, maybe we should have 2-0-stable track master unless something gets introduced that does represent an API change between master -> 2-0-stable?

Or possibly change the .gemspec file so that it depends on 2.x rather than 2.1.x?

[radar]

@sohara I'm actually working on a new PayPal extension. You may have missed the thread about it on spree-user: https://groups.google.com/forum/#!topic/spree-user/zXSL3Y1GAUw. The current extension is really hard to maintain and so I've started fresh.

Could you please take the time to beta test this new extension? Thanks.