swc-windows-installer.py: Rework paths in main()
swc-windows-installer.py: Rework paths in main()

Based on Ethan's path handling in 833d0ee (Add a Nano installer for
Windows, 2013-09-17), while still retaining the entry-point creation
for IPython and nose.

ethanwhite/windows-installer,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,swcarpentry/windows-installer,selimnairb/2014-02-25-swctest,wking/swc-windows-installer,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest