I think we should add an option to select the target EVM version (frontier, homestead, metropolis, etc.)

Maybe this isn't such a big issue anymore for Frontier as everyone moved on to Homestead, but by the time we move on to Metropolis, Homestead will have a much bigger user base.

Key differences:

• Homestead:
  • delegatecall
• Metropolis
  • new gas costs (Long-term gas cost changes for IO-heavy operations to mitigate transaction spam attacks ethereum/EIPs#150)
  • new opcodes (e.g. EVM opcodes: bitwise shifting ethereum/EIPs#145)
  • new behaviour (e.g. EIP 5: Gas costs for return values ethereum/EIPs#8)

Alternatively we might put this into a pragma instead of a compiler option.