SOCIAL-355: Make Connection Serializable

Connection is a field in SocialAuthenticationToken, which implements
Authentication which is Serializable. Therefore Connection must be
Serializable as well.
SOCIAL-355: Fix for OAuth1Connection
Merge pull request #121 from ractive/SOCIAL-355

SOCIAL-355: Make Connection Serializable
Add tests for connection serialization. SOCIAL-355
Fix deprecation warning for Thymeleaf dialect.
Add mavenLocal() repository as fallback when building offline.