Clean up extensions a bit. Removes newest form of registering extensions, more consistently sets and uses an extension's ext_name, and makes a lot of things errors instead of just log messages in hopes that people can't get too far with a messed-up config.
Remove yet another way to register extensions and register/activate FrontMatter like a normal extension.
Fix some references to extensions[:frontmatter]
Remove Asciidoc support in favor of a middleman-asciidoc extension.
Merge pull request #1237 from bhollis/ext

Clean up extensions a bit
Merge pull request #1238 from bhollis/asciidoc

Remove Asciidoc support in favor of a middleman-asciidoc extension
Add options hash to asset methods to allow special options to be passed through from extensions
Update haml and sass deps

Conflicts:
	Gemfile
	middleman/middleman.gemspec