Implement different providers for Bitcoin block headers

[gcarq]

Currently fetchd.py pulls Bitcoin block headers from blockr.io, as this is a single point of failure the script should implement an option to pull from a local Bitcoin node.

[ethers]

Yes, this needs improvement.

Another issue, currently affecting this script is using blockchain.info to get the latest block height:
data = make_request('https://blockchain.info/latestblock')

bci is currently missing blocks for 4hrs:
Home Welcome to Blockchain More...
Height Age Transactions Total Sent Relayed By Size (kB)
428046 4 hours 10 minutes 1647 10,815.28 BTC F2Pool 999.93
428045 4 hours 23 minutes 1493 7,279.14 BTC F2Pool 999.82
428044 4 hours 32 minutes 2434 12,737.90 BTC BitFury 998

[rainbreak]

Another fallback would be to talk to an electrum server.