Added features for bounce on scaling, and disabeling og medium level zoom

Hi, i've added a couple of features to the library, and appended examples in the demo. I hope that you and others will find them useful :-)

1) Setting an upper and lower bound on the level of 'bounce' a PhotoView can do. Previously it came to a complete halt when reaching max, and had no lower limit.

2) A method to disable the medium zoom level. And if zoom level is within 25% of max scale, it will go to minimum scale, instead of the last few percentage to the max scale.