Check to make sure we don't divide by 0

In StickyGridHeadersBaseAdapterWrapper -> unFilledSpacesInHeaderGroup if mNumColumns
is equal to zero we will have a divide by 0 exception. If mNumColumns is zero we just
return 0.
Merge pull request #92 from nbutton23/divide-by-zero-fix

Check to make sure we don't divide by 0
Change README to reflect MinSDK 2.3
Merge pull request #95 from nbutton23/master

Change README to reflect MinSDK 2.3