Allow filename with extension in find_cmd in Windows.

closes gh-3316 (hopefully)
Merge pull request #3338 from takluyver/i3316

closes #3316

According to [documentation here](http://docs.activestate.com/activepython/2.4/pywin32/win32api__SearchPath_meth.html),
SearchPath takes an optional third parameter for the extension,
which is only added if the filename doesn't have an extension.