Supply default values for extension configuration values as required by Markdown 2.5
Print exception & error message if extension creation fails
Return 'root' from PelicanMathJaxTreeProcessor.run as required by the spec in Markdown 2.5
Correct bug where script was being printed as representation of python array rather than actual script
correct bug around configuring mathjax script value
Merge pull request #14 from hydrogen18/master

Fix Issue #13 - Markdown 2.5 compatibiltiy problems