Enhance blockchain stats with proof-of-work metrics and contexts

**Feature description**

Blockchain stats for PoW cryptosystems

**Is your feature request related to a problem? Please describe.**
Initial blockchain stats set for Skycoin did not include values for PoW magnitudes. Other important metrics are not computed.

**Describe the solution you'd like**
Add new blockchain network stats in status view

**Additional context**
Refs #215 #239 #32

**Possible implementation**

New metrics to implement are :

- [ ] Top miner list
- [ ] Miner detail
- [ ] Average value
- [ ] Average fee
- [ ] Difficulty
- [ ] Hash rate

Besides the following generic stats should be added as well

- [ ] Tx per day
- [ ] Ratio , velocity of money
- [ ] Average blockchain transaction value (label for conversion to fiat)
- [ ] Coins transacted across the blockchain (label for conversion to fiat)
- [ ] New coins created (label for conversion to fiat)
- [ ] Miner revenue

... besides ...

- [ ] Put metrics results in context (e.g. measurement time span)