Merge of new validator with tests

Per @gvwilson request (issue #34).

> Make files are back on the plan?
>
Yup - we're building lessons with Pandoc, which has lots of options, and 
there are lots of file dependencies that make lessons more complicated 
than workshop websites.

> For what it's worth, I was able to run it from the pages directory using:
> /../tools/validate_markdown_template.py single index.md home/
>
> And from the lesson-template directory using:
> /tools/validate_markdown_template.py single pages/index.md home/
>
> I've pushed a commit with a much-simplified default assumption: when 
> validating links, it looks for markdown in the same folder as the 
> input file being validated. The old convoluted expression is still 
> there, but just to support unit tests that aren't reading from a file.
>
Thanks much,
Greg