Clean-up

* Moved ISS file out of "source" dir
* Removed WAT setup.exe file (can use GitHub Releases for this)
* Put back in the code that registers it with IE (do we need the
  WATReg.exe and BAT files?)
* Remove Windows version sniffing from that code; it does not seem
  to be necessary.
* Using repo's LICENSE instead of license.rtf (removed)
* Formatting tweaks and build instructions for README
* Use preprocessor to reduce repetition in Inno Setup script
* Added missing space character in uninstall message
* Don't restart closed apps post-install (as we create a new IE anyway)
* Add .gitignore and ignore the generated setup file