[Python] Make API client more pluggable

[geekerzp]

• Allows developers to easily customize APIClient and use it to initialize the API class (e.g. PetApi)

• Allows developers to skip the initialization of a new APIClient as the API class (e.g. PetApi) will automatically construct a new default API client if no client has been initialized). For example, the following line becomes optional:

  api_client = SwaggerPetstore.APIClient('http://petstore.swagger.io/v2')

• Tested without issue

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  [INFO] BUILD SUCCESS
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  [INFO] Total time: 26.713 s
  [INFO] Finished at: 2015-05-25T14:45:45+08:00
  [INFO] Final Memory: 11M/156M
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