Make sure all arguments to dpa_has_achievements can be filtered.

Also, phpDoc tweaks.
Add dpa_get_achievements()

Retrieves a list of achievement posts matching criteria. Use when you
need a list of achievements outside the main loop.
Small change to phpDoc in dpa_get_achievements
Add dpa_get_progress()

Retrieves a list of progress posts matching criteria. Use for getting
progress posts outside of the main loop.
Remove unused extract() from dpa_has_progress