added cssclass latex command

This pull request rips off the recently added `textcolor` latex command to add a more general `cssclass` variant. Usage:

    x+\cssclass{myclass}{y}+z

With this stylesheet definition of `.myclass`...

    .myclass {
        background-color: #5A8;
        border-radius: 5px;
        padding: 4px;
        font-weight:bold;
        color:gold;
    }

...that would output something like this:

![image](https://f.cloud.github.com/assets/249764/511182/531f8862-bdfb-11e2-9f95-ba2a86334d94.png)