Release build (8.1)

Travis CI build log: https://travis-ci.org/JvanKatwijk/swradio-8/builds/474138715/

Included is an installer for windows. The installer, setup-swradio.exe, will install the executable
and required libraries in a user specified (or default) folder.
Note that the installer will call upon the official SDRplay installer to install
dll files, implementing the api to get acess to the SDRplay device