Set standard gas price

• Set standard gas price instead of fast
• Format some big integers as string (for clients)
• Closes #101 and #106

Safe Relay Tracing update

• Biggest improvement is the tracing of internal txs and erc20/erc721 transfer events
• Implement endpoints for estimating sending of txs and creation of Safes using all the available payment tokens.
• Includes #89, #84, #83, #72, #69, #65