Remove gems from Gemfile

The gems are already in the .gemspec
Add RSpec and Nokogiri for tests
Add initial test harness and first test!
Fix a bug revealed by the first test :)

This fixes a bug where the regular expression match was too greedy. If a
list had multiple <code> blocks, the link would be created over the
entire span, instead of just the first occurrence.
Add Travis now, because why not?
Cleanup list_item code a bit (perf)
Allow specifying markdown inside of an HTML block

See https://github.com/vmg/redcarpet/issues/13 for more information and
background. Specifically thank you to @pienkowskip and this comment:
https://github.com/vmg/redcarpet/issues/13#issuecomment-43154826 for the
inspiration.
Remove nokogiri because it is annoying...
Enable xhtml processing
Permit <li><p><code> to be rendered as a link as well
Add better docs for "auto-linking anchor tags" description
Add support for alert-boxes
Minor variable rename to avoid confusion
Remove focus tag (oops)
Not going to do definition lists right now
Merge pull request #4 from hashicorp/sethvargo/more_markdown_goodness

More Markdown Goodness :)