Added filter and action to insert achievement thumbnail on the achievements admin page. Customizations includes thumbnail size, column position, and header title.

Image width or height is constrained to no more than 39px so as to not alter the height of item rows. However, should this size prove to be to small, it can easily be adjusted.
Update functions.php

Integrated thumbnail column into original functions instead of setting up filters and actions to add the column after the fact.
Update functions.php

Added string namespace to Badge column.
Merge branch 'AchievementAdminPageWithThumbnails' of https://github.com/mikebronner/achievements into mikebronner-AchievementAdminPageWithThumbnails
Code style tweaks for new picture column in Achievements admin screen. See #85
Remove redundant echo from call to the_post_thumbnail(). See #85

the_post_thumbnail() echos the output itself.
Rename admin UI "Badge" column to "Featured Image". See #85

Avoids confusion between other admin labels and the "badge" type of
achievement.
Add new styling to admin UI for the achievements table. See #85

* Rename picture column header to "Image".
* Resize thumbnail to 32x32.
* Add new CSS file to set column widths on this screen.
Move dpa-thumb image size registration into main admin class. Fixes #85