fix warpimage and projection = 'hammer' (issue 100)
fix warpimage with projection = 'hammer' (issue 100)
fix bug in previous commit
add hammer example to test for issue 100
raise error message when warpimage is used with celestial=True (issue
101)
Merge pull request #102 from jswhit/master

fix warpimage for projection = 'hammer' (issue 100)