facebook share changes and twitter fix

facebook deprecated share functionality it does only meta level image reading ang titles. 
facebook dialog feed supports same function it requires facbook appid. 
refer the link: https://developers.facebook.com/docs/sharing/reference/feed-dialog/v2.1  
issue no: #33 
Added twitter via and image sharing for twitter. #43 

Added redirect url for facebook. #28 

appkey attribute takes any one social share appkey. if any case twitter,facebook,googleplus all required appkey then we can't depend on single attribute. appkey may available for other social network so went with dynamic appkey.