Fix GitHub avatar URL
- user.services.github.id is only available for the authenticated user
- user.services.github.username is available for all users
Limit getInitials() to first and last initial
- should avoid problems with very long multi-part names
- (e.g. "Jose Manuel Garcia Galvez")
Initials Overhaul
- Bugfixes, address concerns brought up in issues #16 and #17
- Add Avatar.options.defaultType ('image' or 'initials')
- Rename Avatar.options.defaultAvatarUrl to
  Avatar.options.defaultAvatarUrl
- Update README