Update API version in anticipation of v0.3 release (#437)

* Update API version in anticipation of v0.3 release
* Use _version_ across both Unity/Python

python/unityagents/environment.py

         atexit.register(self.close)
         self.port = base_port + worker_id
         self._buffer_size = 12000
-        self._python_api = "API-2"
+        self._version_ = "API-3"
         self._loaded = False
         self._open_socket = False
 
                     .format(str(file_name)))
 
             if "apiNumber" not in p:
-                self._unity_api = "API-1"
+                self._unity_version = "API-1"
-                self._unity_api = p["apiNumber"]
-            if self._unity_api != self._python_api:
+                self._unity_version = p["apiNumber"]
+            if self._unity_version != self._version_:
-                    "of ML-Agents.".format(self._python_api, self._unity_api))
+                    "of ML-Agents.".format(self._version_, self._unity_version))
             self._data = {}
             self._global_done = None
             self._academy_name = p["AcademyName"]

unity-environment/Assets/ML-Agents/Scripts/ExternalCommunicator.cs

     StreamWriter logWriter;
     string logPath;
 
-    const string api = "API-2";
+    const string _version_ = "API-3";
 
     /// Placeholder for state information to send.
     [System.Serializable]
         accParamerters.brainParameters = new List<BrainParameters>();
         accParamerters.brainNames = new List<string>();
         accParamerters.externalBrainNames = new List<string>();
-        accParamerters.apiNumber = api;
+        accParamerters.apiNumber = _version_;
         accParamerters.logPath = logPath;
         foreach (Brain b in brains)
         {