matplotlib.pyplot called on import pandas on trunk
the call added in https://github.com/pydata/pandas/commit/61aac3d0a98f748fd9fd4df9df0ef4bd8849f9b5
added an import of tseries.converter from tools\plotting.py
which in turn loads matplotlib:
from matplotlib import pylab

in rc2, the following worked:
import pandas
import matplotlib
matplotlib.use('qt4agg')

in trunk:
WARNING  C:\Python27\lib\site-packages\matplotlib__init__.py:921: UserWarning:  This call to matplotlib.use() has no effect
because the the backend has already been chosen;
matplotlib.use() must be called _before_ pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

  if warn: warnings.warn(_use_error_msg)

which leads to the default matplotlib backend being used

on first inspection it would appear that the line:
 from matplotlib import pylab
in tseries/converter.py is not required as pylab does not appear to be used.
