add config defaults for MathML output
enable Maruku MathML output if configured

Fixes #240
use Jekyll.logger for printing messages
add a cucumber feature to test for Maruku MathML support

This requires one of the MathML engines to be available.
As ritex seems to be the only one installable as a gem,
take ritex.

As the XML attribute order in Ruby 1.8.7 is random, I found
no other way than using this Regex to make it pass.