Python 3 compatibility fixes

[lelit]

The following changes were needed to successfully use slimit with Python 3 in my own project.

I had to use current (not yet released) ply though, with a tiny fix of mine for its issue 44.

[gforcada]

Meanwhile #102 was merged already. Should this be closed? :thinking:

[lelit]

Probably yes. I switched to https://github.com/calmjs/calmjs.parse months ago, so I can't say if #102 implements everything proposed here, though.

[metatoaster]

They should all be more or less implemented since I cut it from the @lelit fork, however I did make some significant changes as some parts of the fork, such as at places where some new changes were done inefficiently. The other major thing is that that there are a pile of bug fixes that simply couldn't easily be done without correcting some fundamental flaws, such as parsing and the implementation of source map generation. The changelog file has references to all of the issues reported on slimit and the calmjs.parse issue tracker.

Since @gforcada is now here in this thread, if Plone 5 is still going to maintain the existing native JavaScript minification function, the minify print function just had a slight import/function name change if the latest calmjs.parse release is to be used; a brief example is in the documentation.