Add limit for decimals

* Add limit for decimals so that operations (division and root)
  not show too much decimals (the default are only 2).
* User options are now attribute of a object named `MATHMLJS`.
* Update demos.
Create function mi2mn

* Create a function to convert mi elements into mn.
* Change localName to nodeName.
Add power function and others changes

* Add power function.
* Add function to remove mrow that is a negative number.
* Add function to restore negative number.
* Add function opSiblingHash and opChildHash.
* Fix bug in miContextmenu.
* Cleanup the code using mathmlSetupElement.

Note: Probably in the next commit the function to restore
negative number will be generalized to cleanup the code.
Add first github pages
Update proof of concept example.