Doc updates for py25 removal
Replace (most) calls to tornado.util.b() with byte literals.

sed -E -e "s/b\('([^']*)'\)/b'\1'/g" -i '' tornado/*.py
sed -E -e 's/b\("([^"]*)"\)/b"\1"/g' -i '' tornado/*.py
Remove the remaining traces of tornado.util.b.
Remove conditional imports for py25.

ssl, json, and multiprocessing are now ubiquitous.

Also misc pyflakes cleanups (unused imports)
Update autopep8 to 0.8.5 and run it.
Use real class decorators and remove other py25-era workarounds
No more  need to check for 'ssl is None' or 'multiprocessing is None'