lifetime_insights method to get metrics by each post

[kangkyu]

Fb::Post#lifetime_insights method will be a good addition because when calling Fb::Page#posts with with_metrics: true it makes one request too big (it could be a reason to get "error code 1: Please reduce the amount of data you're asking for, then retry your request") to get all the metrics of all the posts, but it can be small requests (but many) if we calling metrics of posts post by post. There seems no error flag or sign of exceeding quota limit because of too many requests, has not been found yet

[coveralls]

Coverage increased (+0.08%) to 98.084% when pulling 746d3ea on post_lifetime_insights into 1e0eedc on master.