This patch allows a JPEG image to be saved with a specific qtables
value (in dictionary format).

Previously, this would throw a TypeError when checking if the qtables
value was actually a preset. By adding an isStringType check, we can
avoid this error.
Merge from master
Merge branch 'master' into jpeg_qtables
Fixed JPEG qtables test
Additional jpeg qtables tests
Added docs for JPEG qtables and subsampling
Merge pull request #714 from wiredfool/jpeg_qtables

JPEG qtables