Fixed Throbber/Loading image.

This patch adds the throbber.gif to the servicestack solution and
default swagger-ui html.

@worknik has rightfully removed throbber.gif from his webserver and
instead moved it into the upstream:
https://github.com/wordnik/swagger-ui/pull/55
Merge pull request #482 from ghuntley/swagger-ui

Fixed Throbber/Loading image.