test_ namespaced JSON-RPC endpoints

We propose the following new JSON-RPC endpoints to be optionally implemented by clients.

The motivation for these new endpoints is to allow the following use cases.

• External integration testing (e.g. web3 style libraries).
• Execution of JSON fixture tests over RPC (grey box testing).
• Build JSON test fixtures.

Endpoints

Enabled by a flag of some sort like --enable-testrpc.

• test_resetGenesis TBD define format for genesis params (use BlockchainTests format as base). -> wipes any chain state when called.
• test_importBlock or test_importRawBlock or test_importRLPBlock?????, does NOT bypass validation/verification.
• test_snapshot - Exact copy of State and Chain database up to last mined/finalized block. -> id
• test_revert - Revert to snapshot denoted by id. Discards any pending state like unmined transactions.
• test_mineBlock - bypass TBD header validation. extraData/coinbase/timestamp
• test_knownAccounts - returns enumerated list of account addresses.
• test_exportFixture - returns a BlockchainTest formatted json body.

mineBlocks() -> mine 1 block
mineBlocks([{}, {}]) -> naively mine n blocks.
mineBlocks([{'timestamp': 12345}]) -> mine blocks for each of the list of header overrides.
mineBlocks([{'pow': True}]) -> mine blocks for each item in list, if `pow == True` in any header params, mine using proof of work.