Allow users to easily override zipcode validation

Just like it's already done with the phone field

[Fixes #3065]
[Fixes #3073]
remove unused variable redefinition [Fixes #3072]
Use Spree.url() to build taxonomies jstree urls

URLs were hardcoded which caused errors when users mounted Spree in a
path other then "/"

[Fixes #3070] [Fixes #3071]
master branch is now 2.1.0.beta
Remove legacy comment from order model [Fixes #3063]
Changed required RubyGems version to 1.8.23 so that Heroku can host it
[Fixes #3077]
Create a CHANGELOG file for each Spree component

It should help us to have a detailed idea of what changes between
versions. Both community and core developers could add an entry for
every relevant change. e.g. deprecated methods, relevant API
improvements, fixes.

Fixes #3062
Merge git://github.com/spree/spree