switching to native NSJSONSerialization class instead of 3rd party json serialization library
Merge pull request #5 from segmentio/master

Replace CJSONSerializer and CJSONDeserializer with native NSJSONSerialization
Merge remote-tracking branch 'upstream/master'
Added "client_api_key" to allow supplemental ids to group requests from
multiple apps

Define cpp symbol CLIENT_API_KEY to include support for client api keys