Fix ST2(python26) compatibility.
Fix ST2(python26) compatibility.

Python26 doesn't support set literals
See #44

timonwong/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer,timonwong/OmniMarkupPreviewer,Lyleo/OmniMarkupPreviewer