KeyGenerator from string

[dgarcia360]

While explaining the metadata / mosaic transactions, I found challenging to follow the examples if we have to refer to the key as random UInt64 value that the dev has to store its meaning in a separate database.

To permit the dev to use NEM as a fully decentralized service without having to store data somewhere else and enhance the usability in the SDK, we could provide with a KeyGenerator that takes a string as an input and returns a deterministic UInt64 object.

You can see how this is possible currently using the NamespaceIdGenerator (https://github.com/nemtech/nem2-docs/blob/master/source/resources/examples/typescript/restriction/RestrictingMosaicsTransfersMosaicGlobalRestriction.ts#L42).

In the docs, we can explain more about the maximum value the string can have to be translated to a valid UInt64 value, as well as if can contain special chars or not.

[contractormario]

See #288

[dgarcia360]

@contractormario Thanks!