Enforces convert.wordsToBytes to only allow for byte input

This pull request removes the (mistaken) ability to use a string in `new HDWallet(seed, network)` by adding an assertion to `convert.bytesToWordArray` which until now would happily accept any input and give a result which may result in the equivalent of a `HMAC-SHA512([0])` master key.
It would also previously give an entropy-crippled master key if a hex string was used due to the addition of Javascript characters `"1"` and `1` being `"11"`. 

This pull request will be resolved in future by a migration of `HDWallet` to a stricter API; and the universal use of `Buffer`.
Until then, this is added as a precaution.