Beging preparing for 0.6.0

0.6.0 aims to remove code debt that we've accumulated over time to
provide first-class support for the latest stuff.

Storage backends:
0.6.x will support only :file and :fog for storage backends. Other
backends (like gridfs with Mongoid) will only be supported via external
gems.  This should help alleviate the confusion caused by having
multiple S3 and Cloudfiles backends.

Rails support:
Rails 3.2.0 and greater will be supported. Due to several changes in
ActiveSupport and ActiveRecord since 3.0 plus deprecations already
scheduled for 4.0, it's just not practical to support every Rails
version in future releases.

Aiming for a release of 0.6.0 alongside the release of Rails 3.2.0.
Depend on mysql2 gem for database testing operations instead of sqlite

SQLite was having incompatibility issues with Ruby 1.9.3.  This makes
for a higher barrier to entry when testing carrier wave locally but
ensure that tests can pass on TravisCI.
Remove some JRuby checks

We're not officially supporting JRuby at this point
Drop rbx support - again
Add failing specs for JSON/XML serialization as it should work
Remove a spec that was designed to run last, but doesn't work because specs don't run in any particular order
Merge branch 'master' into 0.6-stable
Mark xml specs as pending
Merge branch '0.6-stable'
Make JSON/XML serialization consistent within the uploader, override ActiveRecord's serializable_hash to include this in the default record serialization payload, closes #590
Add missing requires
In the example, helper new users by demonstrating how to also remove the test-created uploader and associated files.
Caching fog connections to improve performance
Fix the background color for resize_and_pad
    
when using MiniMagick on a .jpg
Merge pull request #599 from labocho/caching-fog-connections

Caching fog connections to improve performance
Merge pull request #596 from malefactor/master

shows users how to remove generated uploader and associated
Merge pull request #601 from dira/master

resize_and_pad pads with black, not white
add gravity option to resize_to_fill
added list of options/default
add quotes to string argument list