Add default returns for eth_coinbase, eth_hashrate, eth_gasPrice JSON RPC calls

[Bhargavasomu]

What was wrong?

Link to Old PR: ethereum/py-evm#1651

As part of Issue #20 .
Implements eth_coinbase and eth_hashrate functions. The return values are the default values as mining isn't supported by trinity as of now.
eth_gasPrice currently returns the average of the gasprices of all the transactions in a block.

How was it fixed?

The functions were modified/added to trinity/rpc/eth.py

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[Bhargavasomu]

@pipermerriam review please.

[pipermerriam]

This needs special handling for the case where len(transactions) == 0.

[Bhargavasomu]

@pipermerriam please take a look now. I have modified the logic so that if the canonical block has no transactions, we go to the parent block and check the number of transactions that parent block has. If that is also 0, then we keep on going to the parent, till we reach a block which has non-zero transactions.
(This approach should work because, as far as I know, the probability of a block having 0 transactions is quite low. Please correct me if wrong.)

[pipermerriam]

@Bhargavasomu sorry to ask you to toss the work you did, but I'm inclined to just drop all the intelligence from the gasPrice for now since we know it's something that will ultimately be pluggable allowing different engines for computing gas prices.

[pipermerriam]

Given that even this approach has edge cases (like the genesis block when there are no transactions) I'm now inclined to drop this whole implementation in favor of a dead simple approach.

def gasPrice(self):
    return int(os.environ.get('TRINITY_GAS_PRICE', eth_utils.to_wei(1, 'gwei')))

This allows the user to set the price if they need via an environment variable and defaults to 1 gwei. In a subsequent PR we can make this pluggable.

[Bhargavasomu]

@pipermerriam I understand the problem, and have made the code in a simple way. Please review. Thankyou.