URL to Mathimage

A new parameter called 'tex' is introduced to the
specialpage SpecialShowMathImage.
For exammple you can now use
<a href="http://localhost:8080/wiki/Special:MathShowImage?tex=E=mc^2" > </a>
to include an SVG-image of the rendering of the input
E=mc^2 wherever you like.

Change-Id: I6a0b310792db04401bd36c0661db862c8b5d34b1