Retrieve Featured Image and Thumbnail Functions

Added functions to allow easy access to featured image and its thumbnail.
Merge branch 'FunctionsToGetAchievementImage' of https://github.com/mikebronner/achievements into mikebronner-FunctionsToGetAchievementImage
Tidy up code standards and phpDoc from 5da0a96. See #90
Add $attr argument to dpa_get_achievement_image() to match get_the_post_thumbnail().

Filter parameters also updated to match. Fixes #90.
Move dpa_achievement_image functions out of pagination section of the achievements post type template file.