context position tweak

@dinhviethoa please review. 

- if context menu cropped, move context to above of clicked point.
- if upper side & down side  of context menu is cropped, align to bottom.

Sorry about my poor english description.

After
![image](https://f.cloud.github.com/assets/756988/2444688/e56c15d4-ae5d-11e3-8def-a877750befa1.png)

add additional 5px to show boundary.
![image](https://f.cloud.github.com/assets/756988/2444695/1cd19684-ae5e-11e3-9df5-a009f34e2647.png)