Changing from md5 to xxhash and simplifying hasher a bit

Hello nicks, eford1, 

Please review the following commits I made in branch 'azulus-compilerCache'.

df869154bcc1744fb2e599607f5f4888cffaa7c5 (2013-05-21 11:38:21 -0700)
Speeding up / simplifying node hasher a bit

R=nicks
R=eford1