Add 4byte.directory

[jtakalai]

They have a decent API, see e.g. https://www.4byte.directory/api/v1/signatures/?hex_signature=0x144f2f94

They currently list 75,659 signatures while the Parity contract lists 240 (according to https://etherscan.io/address/0x44691B39d1a75dC4E0A0346CBB15E310e6ED1E86#readContract)

[rekmarks]

We should probably consider doing this sometime, although we'd likely have to scrape the data given our usage (see bottom of page here).

GitHub project page: https://github.com/pipermerriam/ethereum-function-signature-registry