Make the MathJax root configurable

Currently, using

MathJax.Hub.Config({root: '...'});

doesn't cause the root to be copied to Ajax.root. There should be a point at the end of the configuration process, but before files are loaded, where the root is copied to Ajax.root. This may be complicated, because the combined configuration file is loaded before user configuration is processed, so that may need to be taken into account.