Added Net.UserAgentString which all WebClients will use
Can set user-agent through cmdline with --net-useragent
Added --net-useragent to NetKAN
Merge pull request #616 from AlexanderDzhoganov/612_webclient_useragent

Code looks good, Travis is happy, engage.....

and I hope that my PR will merge cleanly :)
#403: Allow Prerelease flag for github releases
#403: Forgot to amend unit test