make possible to add new contracts using IPFS metadata

[sponnet]

Add a event + method :

event ContractMetadataAdded(string _metaDataIPFSHash);

[sponnet]

please drag to Q/A when this is done. It seems done to me by ipfsconsortium/IPFSConsortiumProxy#5 - but I'll leave it up to you.

[sponnet]

could you maybe also add your example metadata payload to the README with some explanation ?