Commands no longer match README

[joelburget]

For example, build/bin/istanbul decode ... fails with "No help topic for 'decode'" (same for encoding). You now need to use the extra command and the decode subcommand.

> build/bin/istanbul extra decode --extradata 0x0000000000000000000000000000000000000000000000000000000000000000f89af85494475cc98b5521ab2a1335683e7567c8048bfe79ed9407d8299de61faed3686ba4c4e6c3b9083d7e2371944fe035ce99af680d89e2c4d73aca01dbfc1bd2fd94dc421209441a754f79c4a4ecd2b49c935aad0312b8410000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000c0
vanity:  0x0000000000000000000000000000000000000000000000000000000000000000
validator:  0x475cc98B5521AB2A1335683e7567c8048BfE79eD
validator:  0x07D8299de61FAeD3686BA4c4e6c3B9083d7e2371
validator:  0x4fe035CE99AF680d89e2c4D73aCA01DBFc1Bd2FD
validator:  0xdC421209441A754F79C4A4eCD2b49c935AAD0312
seal: 0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

I would send a PR to change the README to use the subcommands, but it's probably preferable to do a bit more work and change them back to toplevel commands?

[yutelin]

We've added another command to help generate genesis.json. This issue seems to be fixed in PR #89