Use Multiple Post Thumbnails plugin to add post thumbnails

Initial support is for background and thumbnail images on both
posts and pages
Move default image size creation to Spine_Theme_Images
Move default image sizes to Spine_Theme_Images
Introduce spine_the_*_image for background, featured, thumbnail

These functions mimic the behavior of the_post_thumbnail() and
allow for easy display of the post thumbnails in theme templates
Provide wrappers to allow for custom has_post_thumbnail

Introduces spine_has_featured_image, spine_has_background_image,
spine_has_thumbnail_image to easily check at the template level if
a type of post thumbnail is available through the Multiple Post
Thubmnails plugin.
Provide wrappers to retrieve image source on multiple thumbnails

Adds spine_get_featured_image_src, spine_get_background_image_src,
and spine_get_thumbnail_image_src to retrieve the full URL for
images that are added through Multiple Post Thumbnails
Convert existing single post thumbnail code to use new structure
Clarify default sizes for background, featured, and thumbnail images
Add docblock for the_post_thumbnail()
Correct bad method call in Spine_Theme_Images
Merge pull request #63 from washingtonstateuniversity/feature/featured-images

[PR] Add multiple post thumbnails