Deprecate Refinery::Plugin#version.

AFAIK Plugin#version is not used anywhere.
Merge remote-tracking branch 'origin/remove_plugin_version'
Improved caller logic in Refinery#deprecate functionality.
Fixed line ending.

[ci skip]
Use hash-sign for numbered list in "8 - Extending Models" guide
css.empty check for empty array
Merge branch 'master' of git://github.com/refinery/refinerycms into upstream