Added include atomic to types.hpp

Fixes build issue mentioned here https://bitcointalk.org/index.php?topic=259999.msg2865664#msg2865664 and here https://github.com/spesmilo/libbitcoin/issues/12

Still having compile issues on clean ubuntu 12.04 install with all dependencies installed. gcc 4.7.3

http://pastebin.com/JA612Gp6