add .ruby-version
update markitup.js to latest release 1.1.13
prepare for breaking changes:

* update rails requirement to `>= 3.1`
* update gemspec dependencies
* add test dependencies
* remove gemspec cruft
* move version file to namespace-conventional
  location
* bump version to next prerelease
update engine:

* fix incorrect top-level module namespace
* use isolated namespace for engine
* add initial module test
* add Configuration class and tests
* add Rakefile for running tests
update .gitignore
add Gemfile for multi-platform test dependencies
add live preview:

* add PreviewController#preview, view, layout, and
  tests
* add route to mount preview url via the engine
* add test/dummy app for testing engine controller
remove original vendor/assets, prepare to replace:

* the conventions for "Skins" and "Sets" have
  changed. this is house cleaning for the next 
  series of commits which will reintroduce the 
  existing sets and more.
add default `markitup` skin
add `simple` skin
add `xbbcode` set
add `wiki` set
add `txt2tags` set
add `texy` set
add `textile` set
add `rest` set
add `markmin` set
add `markdown` set
add `html` set
add `dotclear` set