Replace CocoaHTTP server with GCDWebServer packaged as static framework.

GCDWebServer is a modern alternative to CococaHTTPServer (last update in 03/2013) with support for Grand Central Dispatch, Travis support and a clean API. Within Cordova plugin projects requiring a lean web server, it has proven to be more stable. Also there are way less issues / edge cases than in CocoaHTTPs  (e.g., not serving preloaded resources). Also, by adding the library as a iOS static framework, the CLI build process tends to be less brittle as XCode does not have to update the XCode project file with a lot of file references every time "cordova prepare" is called.