Introduce Autoprefixer; closes #236

[PixievoltNo1]

As discussed in #236, this replaces prefixing mixins with Autoprefixer, making the Sass source a little cleaner and eliminating many unnecessary prefixes from dist/css/. I got some great results:

• The combined file size of dist/css/ was reduced by about 43%! (59,878 bytes now vs. 105,605 bytes then!)
• dist/css/vex.css in particular dropped 117 lines of code, and some theme files dropped over 200 each.

A few notes on my work:

• As mentioned in the issue, supported browser versions are now formally specified in package.json. Some versions are very old and very unused, but I leave any upgrade decisions to you.
• In addition to Autoprefixer-related work, I removed a rule from vex.sass that was clearly only there to work around the lack of rgba() support in IE <= 8.
• A side effect of my work is that unprefixed versions of things now appear after prefixed versions, which is the recommended practice anyways.
• Most of _mixins.sass is gone; only =pie-clearfix and its dependencies remain. I couldn't figure out if it's still helping anything in IE >= 9.

[bbatliner]

Thanks for doing this! I've looked over the code on GitHub and all of the changes look good. Fantastic results on the filesize reduction of the CSS.

I will checkout your branch on my machine this weekend and doublecheck the build process, make sure everything works together, etc. and merge in after.

Thanks again!