Add the option to make headers ignore horizontal padding. Also fixes typo in measurement.

Hi,

great project, fits my needs nicely. I added the option to make headers exempt from horizontal padding. I wanted my grid items nicely padded on the left and right, but section headers should fill the whole parent.

Also, on line 750, you subtract the padding from the constant instead of the width:

int widthMeasureSpec = MeasureSpec.makeMeasureSpec(getWidth(),
-                    MeasureSpec.EXACTLY - getPaddingLeft() - getPaddingRight());

Cheers,
Kevin