Fix for Python 3.3
There were two problems with a simple 'import pandas' on python 3.3, that have to do with slots.
For the first one, I converted the class initializer into an instance variable.
For the second one, there seems to be a property to compute 'ndim' so I simply removed it.

(I couldn't figure out quickly how to run the tests under Python 3; a number of problems are in the way.)
