Issue title: memory leak
 Issue body: I put a Fragment inside PtrClassicFrameLayout.When I finish my Activity,I found the Activity still in memory.
This is the analyze result from MAT.
![image](https://cloud.githubusercontent.com/assets/8686438/7896611/f12dd244-06f0-11e5-84d9-e36256b04704.png)

 Comments: 
 Comment 0: It seems like a `Message` leak.  There is also another leak which holds the `ViewRootImpl`. 

 Comment 1: Yeah,It's my fault.I call refreshComplete directly(start time is zero),so it has a super long time deply message
{'source': 'GithubIssues', 'repo_name_with_owner': 'liaohuqiu/android-Ultra-Pull-To-Refresh', 'issue_id': 82761565, 'repo_stars': 10753.0}