Add sstorage p2p protocol -- part 2

[ping-ke]

doc about the sstorage p2p protocol, https://docs.google.com/document/d/1rfuGeCBU1Qj52gPOSyhefgb8hqLooTNPGiAeZbc85u4/edit; and the part 1 code is https://github.com/QuarkChain/go-ethereum/pull/100. Part 2 only contains the p2p sstorage protocol usage code.

• eth/
  • peer/peerset/handler/handler_sstorage
  • add sstorage peer to peerset
  • add sstorage handler to eth/handler
• eth/downloader/
  • add sstorage sync
• eth/ethconfig/
  • add sstorage config