Replace icons.

* Add @3x icon for iPhone 6.
* Optimize png.
Add "- (id)initWithPerformIfLineNotInstalled:(BOOL)performIfLineNotInstalled" initializer to LINEActvity.
Merge pull request #8 from yhirano/master

Replace icons and add [LINEActivity initWithPerformIfLineNotInstalled:(BOOL)performIfLineNotInstalled] initializer.