Direct SQL serialization

Added SQL functions that returns proper json

condenser_api:

• condenser_api.get_transaction
• condenser_api.get_ops_in_block
• condenser_api.get_account_history

account_history_api:

• account_history_api.get_transaction
• account_history_api.get_ops_in_block
• account_history_api.get_account_history
• account_history_api.enum_virtual_ops

Addresses issue #15 (closed) This is also prerequisite for next stage of PostgREST integation (which will mostly reuse here done implementations).

rebased onto: !11 (merged)