API cleanup [WIP]

The API methods have inconsistent path and argument naming, and their response json formats also have inconsistent names and values.

This work needs to be done in its own branch, and then coordinate with known API consumers to update prior to release.

Known defects collected here:

- [x] `/explorer/getEffectiveOutputs` should be `/explorer/coinSupply`, or anything else better.
- [ ] `/outputs` value for "hours" is an int, but "coins" are a string. The value for "hours" should be a string
- [ ] `/uxout` returns integers for both "coins" and "hours". The "coins" integer value is multiplied by 1000000 (seemingly to support decimals). This does not match other APIs.
- [x] `/transaction` "status" element contains both "confirmed" and "unconfirmed" booleans
- [x] `/spend` errors should use HTTP status codes
- [ ] `/wallet/balance` should return strings for coins and hours
- [ ] Clean up transaction "time" field (https://github.com/skycoin/skycoin/issues/1153)
- [ ] Rename block head "block_hash" to "hash" and "previous_block_hash" to "previous_hash"