Adding APIs for test users

[theeluwin]

I found that there's get_app_access_token function for app access token but no any further APIs like creating or accessing test users.
I'm thinking of implementing all APIs from https://developers.facebook.com/docs/test_users/.

[theeluwin]

I was working on this but I'm not really good at writing nice commit message nor PEP8 style. Also, there can't be any appropriate unit test for this feature.

[martey]

I'm closing this. Further discussion on this feature should take place in the comments at #120.

[theeluwin]

okay. thanks

[martey]

I am reopening this because I deleted the branch that #120 referred to. This seemed to cause #120 to automatically close. Since I cannot reopen it, further discussion should go here.

I have reviewed the code and do not see any obvious problems with it. I will write unit tests for it and merge it in.

[theeluwin]

Thanks!!

[martey]

Closing in favor of #172.