proper rendering of the Beta function

printing out an expression involving the beta function would display it in terms of the gamma function, both in an IPython notebook (screenshot below) and in the console. This fixes that, using a non-italicized roman upper case "B" which is a function of two arguments.

![image](https://cloud.githubusercontent.com/assets/573848/3791415/7ba58d24-1b3b-11e4-8827-523f601e2c2f.png)