Doc - Web3Config.set_provider

* Populus v1.10.0
* OS: linux

It looks like official documentation about configuration has some confusion.
It takes me some time to even figure out how chain configuration works.
- set_provider is missing, maybe it should be set_provider_class?
- in the other side default ```populus init``` generates ```populus.json``` with ```populus.chain.geth.TestnetChain``` while in doc we see ```populus.chain.TesterChain``` that works better in my case.

* AttributeError: 'Web3Config' object has no attribute 'set_provider'
* web3_config.set_provider('ipc')
* example from documentation: http://populus.readthedocs.io/en/v1.10.0/config.html

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