web3_sha3 not working under Python 2.7 on MacOS

Dear all,

i encountered some problem with the web3_sha3 rpc method on Mac OS with Python 2.7. Actually, line 150 in client.py does not reaturn a hex string starting with 0x. Since i assume that this is a Mac specific problem, otherwise other should have encountered it also, i would suggest to add something like:

if not data.startsWith('0x'):
data = '0x' + data