Adding markdown handlebars helper. Updating example.

Adding markdown handlebars helper to render markdown
when rendering handlebars templates.

Updating example to include a markdown handlebars helper
example.
Updating markdown handlebars example

Updating the markdown handlebars example to show how handlebars
can be used within the markdown. The handlebars is resolved before
the markdown is converted to html.
Merge branch 'master' of https://github.com/sellside/assemble into sellside
Merge pull request #51 from doowb/master

Markdown handlebars helper