Revert removal of host information in Heartbeat.

Effectively, the host for all heartbeats was the empty string. There is
a strong possibility this caused errors when initializing a
blockchain.
Fix rendezvous hashing and add a test case that failed before
Merge pull request #34 from c00w/rendezvous_hash

Fix rendezvous hashing and add a test case that failed before
Merge pull request #33 from Jonbeek/Fixes

Fix Heartbeat not containing Host information
Merge pull request #32 from c00w/stateinformed

Make the blockchain simpler and add benchmarks
Merge remote-tracking branch 'Sia/master' into stateinformedv2
Fix whitespace of src/common/hash.go