Exclude UserDict when deep reloading NumPy.

[bfroehle]

This prevents another test failure in IPython.lib.tests.test_irunner_pylab_magic::

Traceback (most recent call last):
...
  File "/usr/lib/python2.7/weakref.py", line 53, in __init__
    UserDict.UserDict.__init__(self, *args, **kw)
TypeError: unbound method __init__() must be called with UserDict instance as first argument (got WeakValueDictionary instance instead)

Closes #1636.

[bfroehle]

Overall this seems quite fragile, but suffices to fix the currently failing test on my system (Ubuntu 11.10).

In general, I think it'd be better to either:

1. Isolate dreload(numpy) in its own subprocess; or
2. Remove the dreload(numpy) test altogether.

[fperez]

@bfroehle, I'll try to isolate that in a separate test group, but given the current design of the testing code, that's a little tricky to do. In the meantime, this fixes a broken master test suite, so it's pretty high-priority to merge it; I hate having a failing master suite, b/c it easily masks other new failures.

Merging now, thanks!

[takluyver]

Since this change, an additional test failed on Python 2.6. Hopefully it's just another chance failure, as it looks unrelated, but if it fails again tonight we'll have to look into it:
https://jenkins.shiningpanda.com/ipython/job/ipython-multi/PLATFORM=debian6,PYTHON=CPython-2.6/75/testReport/junit/IPython.zmq.tests.test_message_spec/test_complete/test/