Allow processing some styles in foreground, some in background

Detect when only_process is set but different on has_attached_file and
process_in_background. In which case run foreground styles in normal hook,
and allow the backgrounded job to handle the styles on process_in_background
as normal.
Added test for split_processing option and explanation in README
Use ruby 2.0, p195.

Fixes many problems with rbenv users. rvm should be unchanged.