Fix #78: Make autoprefixer run in .tmp regardless of build type (/concat...

... and /css)

Squashing a few old commits:
Move autoprefixer after cssmin so that /dist will be populated. Another option would be to point autoprefixer:dist to the .tmp directory; this would make it analogous to autoprefixer:server and could reduce redundancy. Either works fine :)
Merge :dist and :server into single :files task that checks both /concat and /<%= cssDir %> (turns out this was the main problem in #78)
Switch autoprefixer:files to autoprefixer:dist