docs and implementation of NewWallet and Wallet.GenerateAddresses need a review

[isaacvr]

While trying to use skycoin/services#6, with the seed, generated by skycoin/services#12, I have found following issues:

Docs for wallet.NewWallet claims that wallet generates a random seed if seed is "". Also it does not mention which encoding is expected.

However:

1. wallet.NewWallet returns error if seed is "" - https://github.com/skycoin/skycoin/blob/feaab10d4f951b8ffea53ef2d37a4cf5e3450b17/src/wallet/wallet.go#L80-L82

2. Wallet.GenerateAddresses does not check for empty seed and on the first pass does not generate new one. Also, on the first pass it converts seed directly to byte representation, while the rest of the code assumes that seed is hex-encoded.
   https://github.com/skycoin/skycoin/blob/feaab10d4f951b8ffea53ef2d37a4cf5e3450b17/src/wallet/wallet.go#L212-L225

3. wallet.NewService uses bip39 to generate wallet's seed and does not encode it to hex.
   https://github.com/skycoin/skycoin/blob/feaab10d4f951b8ffea53ef2d37a4cf5e3450b17/src/wallet/service.go#L52-L61