Add new widget: Featured Achievement

This widget's came back from the 2.x versions of Achievements; say hi!
Add new core function to register custom image sizes
Set alt= attribute for featured achievement widget image
Disable paging on Featured Achievement widget query
Make featured achievement widget use get_posts instead of a WP_Query

For better query optimisation... the previous SQL query didn't have a
LIMIT.
Reorder parameters because I have an alphabetical order thing.
Add new photo grid widget

This brings the "Available Achievements" widget back from the 2.x
versions, under a different name.
Fix incorrect links on the imgs in the Photo Grid widget
Update bbPress credits
Increase priority of achievement CPT metabox
Add support for achievement categories. See #91.

Props mikebronner for idea and original patch
Remove the "categories" submenu from inside the achievement post type menu
Add unit test skeleton files. See #95
Add first WP-CLI command: wp achievements list

http://wp-cli.org/
Remove unused 'filter' param from new default WPCLI param
Rename WPCLI class
Sort 'wp achievements list' command by post_title

Better readability for humans than the post_date, which isn't displayed.
Add new WPCLI command 'achievements-user' for listing achievements unlocked by a specific user
Bump versions and readme to 3.3
Update POT for 3.3