issue 3434: warnings from matplotlib when starting isympy

the issue was that matplotlib.pyplot was being imported before
pylab was initialized. initializing pylab after the import of
matplotlib.pyplot caused a warning to be printed to the console.

fixes issue 3434. on my box, this also reduced the import time of sympy from 2.0s to 0.3s. If other people encounter a similar speed-up, you could consider issue 3427 fixed as well.

refs:
https://code.google.com/p/sympy/issues/detail?id=3434
https://code.google.com/p/sympy/issues/detail?id=3427