Alternative preview using 'matplotlib'

Hi, this is my first stab at implementing an alternative for the preview function, which does not rely on having LaTeX installed, but rather uses the `mathtext` module from `matplotlib`. Mailinglist thread: https://groups.google.com/forum/?hl=de&fromgroups=#!topic/sympy/zuvT11mYeso
This is probably not suitable for merging as is, since I'm new to Python, and very new to `sympy`. However, I am eager to learn, and would highly appreciate the opportunity to do so. So please make suggestions and critize, and I will try my best to improve the code.
Regards
Psirus