Fixed signal.get_window with unicode window name

[zeehio]

This minor fix allows unicode objects to be used in scipy.signal.get_window.

This code does not fail anymore (python2):
from scipy.signal import get_window
get_window(u'blackman', 32) # This failed because u'blackman' is not a string on python2.

This code does not fail anymore (python2):
from future import unicode_literals
from scipy.signal import get_window
get_window('blackman', 32) # This failed because u'blackman' is unicode

[coveralls]

Coverage remained the same when pulling 371ee16 on zeehio:master into 6761b2c on scipy:master.

[coveralls]

Coverage increased (+0.0%) when pulling 371ee16 on zeehio:master into 6761b2c on scipy:master.

[rgommers]

This fails with numpy 1.5.1:

 File "/home/travis/build/scipy/scipy/build/testenv/lib/python2.6/site-packages/scipy/signal/windows.py", line 7, in <module>
from numpy.compat.py3k import basestring
ImportError: cannot import name basestring

[coveralls]

Coverage decreased (-0.0%) when pulling 8e0f0ea on zeehio:master into 6f9b0a7 on scipy:master.

[zeehio]

@rgommers I am sorry. I did not account for all the combinations of python and numpy and I did not have travis set up. Now that all travis tests pass successfully. Thank you for taking a look into it.

[argriffing]

I'm not too familiar with python 3, but scipy has a forked version of six.py. Would it make sense to use scipy.lib.six somehow?
https://github.com/scipy/scipy/blob/master/scipy/lib/six.py

[coveralls]

Coverage increased (+0.0%) when pulling d120ea4 on zeehio:master into 6f9b0a7 on scipy:master.

[zeehio]

I am not familiar enough with the scipy code, thanks @argriffing. Now the diff is much cleaner :-)

[pv]

Thanks, looks ok. Regression test would be useful, though.