tables: add new package, version 3.1.1

From the project page [1]:
> PyTables is a package for managing hierarchical datasets and designed to
> efficiently and easily cope with extremely large amounts of data.

It offers read/write access to hdf5 archives through a hierarchical interface.

I am adding it, because pandas depends on tables for hdf5 read/write
functionality.

Note: The name is a bit of an issue. The project is called pytables, whereas
the python module is simply called tables. I chose to name the nix-expression
tables for consistency with the source-tarball and the module name.

[1]: http://www.pytables.org/moin