Use prototypes for model methods

[delenius]

Fixes #2044.

[wing328]

@delenius Test result looks good.

    #defaultHeaders
      ✓ should initialize default headers to be an empty object
      ✓ should put default headers in request
      ✓ should override default headers with provided header params

  PetApi
    ✓ should create and get pet (1328ms)


  18 passing (1s)

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 12.924 s
[INFO] Finished at: 2016-02-07T15:31:05+08:00
[INFO] Final Memory: 7M/245M
[INFO] ------------------------------------------------------------------------

PR merged. Next time please update the Javascript Petstore sample as well (ref: https://github.com/swagger-api/swagger-codegen/blob/master/CONTRIBUTING.md).