Fixed #721

Testing for bots should be case insensitive - facebook bots are not all
lowercase - they adapted CamelCase on some servers. I've added the
linkedin bot just for the sake of it.

*Text from first PR:*
Spiderable failed when facebook bot tried to fetch without parametres eg. `http://meteorrain.meteor.com` - Spiderable now depending on the reg.ex to catch the bot user agent.

The reg.ex failed to catch this due to a change in facebook bot header. Header going from low case to CamelCase like the `LinkedInBot`

* Fixed by add 'i' to the reg.ex
* Added linkedinbot

*Tested in facebook and linkedin*