Move to plugin instead of widget

MathML is incompatible with widgets because widgets require
extra tags that make some features of MathML not work.

* Change Makefile
* Change API
* Improve dialog

BUGS:

* Plugin icon wasn't load
* MathML tags is stripped (look like this is due CKEditor:
  http://ckeditor.com/forums/Support/unfilteredhtml-mode-of-editor.insertHtml-not-working)
Improve sample

Add some default text in the sample.
Add edit previous MathML feature

* Add some skeleton
* Retrieve display status

TODO:

* Retrieve (La)TeX
Update README

The README was update to the plugin version.
Fix bug to insert MathML

Use a hack found in http://stackoverflow.com/a/17339275/1802726.
Add RTL option
Add edit old equation feature
Improve in examples

- Add (La)TeX version of equations
- Remove duplicates
- Add namespace
- Fix inline example
Update README