Parse tex strings

Some refinements of the function `parseTexSubstring`.
The addressed problems are:
- `\text` overrides style settings like `bf` that are made outside.
- `\fontname` inside matlab uses fontnames of the system, which might not be compatible with tex

Especially, the fontnames cause problems. My current solution hard-codes translations of font-names into tex commands. Until know, the list of supported entries contains only one entry (yes, I admit, that I was driven by `testfunction(76)`). 
This list should definitely be extended in the future, unless someone knows a better way to deal with this problem.

Another issue is, that style commands like `\bf` only take effect to the end of the `\text` command, but previously, they had no effect.

Still, the gain of this pull request can be seen in the following figure. I merged the code with #397 to create this figure, because the gain can only be seen in the later lines.

![fontsandstyles](https://cloud.githubusercontent.com/assets/2790591/4902772/bd45f9e6-643b-11e4-811e-8c4104c467cb.png)