Added continuous integration

[yallie]

Hello,

I've taken the liberty to update all referenced Nuget packages to their latest versions and add the continuous integration settings to the project (both Travis-CI and Appveyor). Also, I've made these minor changes:

• Removed numerous Send method overloads from IClient and IHost interfaces
• Replaced the overloads with the equivalent extension methods to keep the same API
• Added copyright headers to the interfaces, same as with other files
• Removed extra src folder (src/src/MessageWire.sln -> src/MessageWire.sln)
• Added .editorconfig file so that Visual Studio 2017 picks up the project's tab settings automatically
• Added badges to the README to display the current build status and the package version like this:

For some reason Github UI doesn't display all the file renames properly (although Git itself detects them just fine). I'm a bit disappointed that the diff looks like I've made a lot of changes — there are not that many changes, actually.

Last, but not least — thanks for this project! Hope it's not abandoned! :)

Regards, Alex