Add testnet support.
Merge pull request #28 from christianlundkvist/testnet_support

Expand testnet support using blockr APIs.
Change flag from bool to string.
Merge pull request #29 from christianlundkvist/testnet_support

Change testnet network flag from bool to string.
Fixed changebase for cases (b,b,minlen)
Merge branch 'master' of github.com:vbuterin/pybitcointools