Add support for testnetworks (rinkeby/kovan)

[Anindya-Prithvi]

I tried testing karl on my testing contract on rinkeby, it returned "extraData" too long.
I have a fix for that as seen on ethereum-etl/issue-178

from web3.middleware import geth_poa_middleware
self.web3.middleware_stack.inject(geth_poa_middleware, layer=0)

And it works like a charm

[caiosabarros]

in which file do I insert this? @Anindya-Prithvi

[Anindya-Prithvi]

@caiosabarros please see the mentioned pull request, I have patched it already