Also creates a ZIP for non-Admin (per-user) installs.

WIX requires the license file to have a .txt or .rtf extension so I
added the .txt extension. I've taken the opportunity to migrate the 3rd
party licenses to a COPYING subdirectory and have added licensing
details to the README.md file.

To build the installer, install WIX and simply run `cpack -C Release`

Also removed the explicit --config option from the
clamav-clamonacc.service file because it should not be required and
isn't being generated correctly when using autotools anyways, especially
after changes in this commit.