bug fix on LinkedInShare::isValid() method

- bug fix on LinkedInShare::isValid() method
- added style in public part for connection button
- fixed indentation problems
clean session data just invalidate LinkedIn data

instead of destroying the whole session, now just set
'linkedin_session_data' to null
linkedin-signin-button.png
linkedin-signin-button.png
Merge pull request #2 from brainsonic-dev/master

LinkedIn data invalidation and authentication button style