Add other blockchain APIs

[dnpotter]

Currently only blockchain.info is supported. Multiple blockchain APIs are needed to ensure availability of the OpenSig service.

• Needs a strategy for selecting an api on startup, including, perhaps, a dynamic availability calculation that can react intelligently to an APIs short and longterm service interruptions.
• Needs to allow configuration of which apis to use, including the users own apis.

[dnpotter]

APIs available: https://blockexplorer.com/api-ref (open source server based on bitpay's code: https://github.com/bitpay/insight-api) http://blockr.io/documentation/api