add text rotate parameter

[MichalPP]

add parameter to rotate text to fix bug #80

[meyfa]

Thanks for wanting to help, always appreciated. Unfortunately, I cannot merge this PR. Reasons:

• The proposed change is making use of an attribute rotate2, which is not part of the SVG specification.
• The code contains double quotes / string interpolation, which should be avoided.
• But most importantly, this is way too specific. It disregards preexisting transform attributes and style properties and transforms on elements other than texts.

Note also that the setRotate method should probably be setTransform and should be located higher up the inheritance chain, as it is not specific to text elements.

[MichalPP]

I did not know how to store a parameter, so I misused a non-standard non-used SVG parameter (ugly hack).

the new setRotate f: SVG and GD use different centers of rotation. for SVG output, I have to manually set the center of rotation at the beginning of the text. to avoid retyping of the same XY coordinates, I've created an extra function. plus GD rotate is -1 times SVG rotate.

afaik, there is no basic rotate parameter in PHP GD output apart from texts. eg, to rotate a rectangle, one would have to calculate the points of SVG rectangle, rotate and use imagefilledpolygon. skewing is yet another story :(

I don't think a generic setTransform will be usable to generate both SVG and PHP GD outputs.