Mac compatiblity

There were few issues on compatibility with mac osx. I have made some small tweeks and now this works fine on my macbook pro with OSX 10.9.4. 
 
--> Tweeked the way ImageTk is imported form PIL.
--> instead of using imagemagick's convert for converting PDFs to PNGs I have used sips which is native to OSX.

Now this is working fine on mac. Please pull these changes if you think it solves your problem.