achievements 7.x-1.6

Changes since 7.x-1.5:

• #2277217 by jhedstrom: Ability to define achievement as manual-only
• #2268713 by jhedstrom: Contextual links for granting/taking achievements on a user page
• #2225971 by jhedstrom: Remove achievements_recent code
• #1578198 by webflo: Strict warning: Only variables should be passed by reference in achievements_user_page()
• #1788800 by jhedstrom, cr0ss: Account->name should be turned to format_username()
• #1482852 by Fabianx: PHP 5.3, Call-time pass-by-reference
• #1820556: Don't show achievement popups if the overlay is active.
• Adding support to the primary leaderboard functions for nicer code reuse.
• Early draft of achievements_recent: pre-filling the recent leaderboards supported.
• Stubbing 1.6 release.