Addition of control over css margin-top and margin-bottom fields

Hi Matt,
It would be nice to be able to minimize the whitespace above and below BlockGrids to avoid having to do this sort of thing:
http://nbviewer.ipython.org/urls/raw.github.com/ctb/2013-pycon-awesome-big-data-algorithms/master/04-bloom-filters.ipynb
Here I assume Titus Brown has made the show() method in his BloomFilter class more complicated than I think it should be, because by default there's a large amount of whitespace above and below BlockGrids, which is especially noticeable when they are 1D. This pull request achieves this change.
Compare the result here, where I used my modified version of ipythonblocks: http://nbviewer.ipython.org/5180685
Even if you don't want to merge my contribution, you might like to look at the code, because I changed some private fields in your existing Block class that looked wrong to me, i.e., red, green, blue -> _red, _green, _blue. These probably aren't accessed properly in the current release code. I should have separated those into their own changeset but I was lazy.