API Changes for android

Restructuring repository. Apparently it's impossible to build the android client (from outside of this repository, as part of a multi-module project, see bitmaskcore) if the client lib depends on go source files in the parent folder of ./client. This is why a new package obfsvpn was created which is on the same directory level as ./client. Also the server source files have been moved to a server package, similar to the the client code package structure.