register embedded engine for asciidoc

- asciidoc content handled by Asciidoctor by way of Tilt 1.3.4
- document embedded asciidoc engine to README.md
- add asciidoctor as development dependency
Merge pull request #357 from mojavelinux/asciidoc

register embedded engine for asciidoc
readme cleanup

[ci skip]
changes

[ci skip]
test rails 4 beta1
move plugin documentation to separate files
remove plugins from literate tests
require logic less if option given