Hard dep on python lex, Spacebar Preview of slide.

This is a tiny pull request which just includes 2 simple commits; 

 ee28ac1 Hard dep on python lex libraries, required to make. 
 013b866 Potential fix for issue #48 - Spacebar Preview of Slide.

I didn't branch to separate these two issues because they are so small, but I'll try that with my next PR :)