Fix the position of contextMenu for button_add button for mobile device

This pull request fixes the position of context menu for button_add on mobile device which should be shown on the top right like desktop version.
The root cause is touchend event will return no pageX and pageY, and thus we need to capture pageX and pageY from orignalEvent.

It is ok to show the context menu on the top left, but I think we should give the user least surprised.
I am not sure if this change will have some side effects.
Please help review if this change is ok.

Before change:
![2014-10-10 12 40 31](https://cloud.githubusercontent.com/assets/168433/4592834/615699ca-5080-11e4-8e47-7ea47c1c853a.jpg)

After change:
![2014-10-10 12 43 57](https://cloud.githubusercontent.com/assets/168433/4592838/67536452-5080-11e4-9206-8f242e10d9c3.jpg)

By the way, the wierd white shadow on the context menu can only be reproduced on my Nexus 4 with latest Chrome.
It seems like the "border-radius and box-shadow" make this, because when I disable one of them, the white shadow is gone.
But I have tried many devices:
- iPod touch 5 with Safari and Chrome
- Nexus 5 with Chrome
- LG G Tablet 8.3 with Chrome

The issue is not happened.

So I am not sure if this is only happened on specific devices and don't know what's the root cause.
Maybe we can track this issue if other people also have the same issue.