add mathbb macro

okay, this is quite some changes
but I think in this way it also possible to add mathcal, and mathfrak easily

I've implemented it so that it also accepts letters, so that \mathbb{REAL} works, but \mathbb{A+B} wont work. I LaTeX self, you get strange ouput if you do something like \mathbb{R^n} for example:

<a href="http://www.codecogs.com/eqnedit.php?latex=\mathbb{R^n}" target="_blank"><img src="http://latex.codecogs.com/gif.latex?\mathbb{R^n}" title="\mathbb{R^n}" /></a>

I think it is better to just give an error message, saying that it expected a letter.