Merge of new validator with tests

Per @gvwilson request (issue #34).
The big assumption is that every lesson repository contains its own copy of the validator. Rather than try to relate the /pages folder to some arbitrary place where the script is run from, it's hardcoding an assumption that markdown files are in a certain place relative to the script file.

To clarify: what is the assumed use case? Are we always validating a lesson that exists in a particular repository structure? If the user is allowed to specify a directory to analyze, what directory should that be? (lesson root, markdown folder, etc)