Work around bug with pngcrush -ow (bug 983426)

Also add -q option to keep things quiet, and rename to pngcrush_image
since it's not specific to icons - we'll use it for previews soon.
Merge pull request #1926 from diox/fix-pngcrush-errors

Work around bug with pngcrush -ow (bug 983426)
Fix mkt.developers.tests.test_tasks.TestResizePreview