[Java okhttp-gson] Add support of file downloading

[xhh]

• For endpoints with file response, save file from response body and return it
• Allow customizing the folder for file downloading via ApiClient#setTempFolderPath

Integration test result of the Petstore sample:

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 T E S T S
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 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.client.ApiClientTest
 Tests run: 11, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.379 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.client.auth.ApiKeyAuthTest
 Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.01 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.client.auth.HttpBasicAuthTest
 Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.034 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.client.ConfigurationTest
 Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.397 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.client.StringUtilTest
 Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.01 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.petstore.test.PetApiTest
 Tests run: 9, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 23.412 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.petstore.test.StoreApiTest
 Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 9.785 sec
 Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
 Running io.swagger.petstore.test.UserApiTest
 Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 6.889 sec

 Results :

 Tests run: 34, Failures: 0, Errors: 0, Skipped: 0

 [INFO] ------------------------------------------------------------------------
 [INFO] BUILD SUCCESS
 [INFO] ------------------------------------------------------------------------
 [INFO] Total time: 59.264 s
 [INFO] Finished at: 2015-09-13T21:05:24+08:00
 [INFO] Final Memory: 20M/173M
 [INFO] ------------------------------------------------------------------------