add SIToastViewGravityNone;
fix layout bug
change the return type of class methods
add tap support
add missing method definition in header
add shadow opacity property
fix a typo
make layout code more responsive, related to font size
Merge commit 'b3cc90705c029dfeea7a3d3d846217d63158cf38' into develop

Conflicts:
	SIToastView/SIToastView.h
	SIToastView/SIToastView.m
	SIToastViewExample/SIToastViewExample/ViewController.m