Expand testnet support using blockr APIs.

This pull request is to expand testnet support using the recently added testnet capabilities of https://blockr.io.

Using blockr, we can now get unspent outputs from and also push transactions to the testnet. This allows pybitcointools to have the exact same capabilities on mainnet and testnet so that the user can for instance play with multisig, P2SH etc using testnet coins.

Also updated `address_to_script` and `script_to_address` with support for testnet P2SH addresses.