Validate that websites are absolute URLs

with a schema
WCMS-160 Make the Membership model polymorphic.
Changed the name from objectable to unitable. Also fixed the failing test.
Merge pull request #62 from biola/validate_absolute_website_urls

Validate that websites are absolute URLs
Merge pull request #64 from biola/polymorphic_membership

Polymorphic membership
Merge branch 'master' into profile_slugs