Bless Gollum with the power of GFM

With these changes, the Markdown rendered in Gollum wikis will have the
same quality and safety as the MD we render everywhere else @ GitHub.

This commit also changes the old `markup_class` accessor to
`markup_classes`, allowing users to specify a custom Markup class for
each markup language.

gollum.gemspec

@@ -37,6 +37,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency('shoulda')
   s.add_development_dependency('rack-test')
   s.add_development_dependency('wikicloth')
+  s.add_development_dependency('rake')
+  s.add_development_dependency('redcarpet')
 
   # = MANIFEST =
   s.files = %w[