add new @Headers and @Header annotations

fix issue #198: add new @Headers and @Header annotations

tested with the below client:
```
  @Headers({
    "API-Version: 3.3.8",
    "Content-Type: application/json"
  })
  static interface DynECTSession {
    @POST("/Session")
    Session login(@SingleEntity Credentials creds);

    @DELETE("/Session")
    Void logout(@Header("Auth-Token") String token);
  }
```