allow external scroll listener
onMeasure throw a NullPointerException if not settled the adapter
Merge pull request #8 from ffgiraldez/onscroll

Allow external scroll listener.
Merge pull request #9 from ffgiraldez/measure_nullpointer

Ignore null adapter when measuring.
Update Library/src/com/tonicartos/widget/stickygridheaders/StickyGridHeadersGridView.java

Tidy up merged code.
Make section headers touch interactive. Closes #10.
Demonstrate use of touchable section headers.
Improve scroll performance.

Removed unnecessary check on move action event.
Removed debug output.
Use toast for demonstration of header interaction.

Also removed some old debug lines.
Recycle style attributes typed array.