Show caller when asking for readout permission

Get stack and show the caller when asking for readout API permission.

Related to enhancement request to show source where canvas is nested.
https://github.com/kkapsner/CanvasBlocker/issues/8

![clipboard01](https://cloud.githubusercontent.com/assets/965580/5329928/16f172b0-7d99-11e4-80b7-541095a2197b.png)
![clipboard02](https://cloud.githubusercontent.com/assets/965580/5329929/1ad419fa-7d99-11e4-9751-39845c6e6f8f.png)