Minor change to CleanCommand so build works with stdeb
Hi Wes, i've been following your pandas work for a while, and regularly install it on my ubuntu based systems.  I generally use the stdeb (https://github.com/astraw/stdeb) package for creating .deb files from python packages.  Unfortunately, some of your recent changes in setup.py in pandas caused a problem with building using stdeb.  Essentially the problem is that debhelper tries to run "python setup.py clean -a" at some point, which doesn't work because the customized clean command does not have a "-a" option.  I just added a dummy argument "--all, -a" argument that has no effect.

No worries if you'd rather not merge it, just wanted to check.

thanks
