Added tests for nikola.plugins.compile_markdown.
Refactored Nikola-specific Markdown customization into "mdx_nikola" Markdown extension.  Added mdx_gist.py and test cases.
Make docttest for mdx_gist work from nikola.plugins.compile_markdown.mdx_gist.
Fix exception-handling for Py3k.
Merge pull request #380 from mrabbitt/feature/gist_markdown

Added inline gist for Markdown, refactored markdown extension handling.