Fix error in summary warning

Summary was not updated to reflect variables name change (which is
caught by 'test_summary_2dim_value_model').
Revert "Move summary to stats module"

This reverts commit 6dda7e1ab641a40336bd86d53201ddbefe47c5e8.

The is being reverted to make it easier to compare changes with the
master branch.

Conflicts:
        pymc/__init__.py
        pymc/stats.py
        pymc/tests/test_trace.py
        pymc/trace.py
Revert "Rename sample.py to sampling.py"

This reverts commit 683507b08504c751a32dcc7e12bc2711bfbe2ecc.

The is being reverted to make it easier to compare changes with the
master branch.

However, this reintroduces the naming conflict that the commit fixed.
'pymc.sample' now refers to the function, but the module of the same
name cannot be accessed. This makes it difficult to test anything other
than the top-level functions that are imported into pymc/__init__.py
(sample and iter_sample).

Conflicts:
        pymc/__init__.py