Generate Random Mnemonic

Chained on #3.

This pull request changes `generateMnemonic` to use [`secure-random`](https://github.com/jprichardson/secure-random/blob/master/lib/secure-random.js) instead of `globals.crypto`.

However, while this test is extensive enough, I feel it is not relevant to this module, and would actually suggest an alternative PR where `generateMnemonic` is removed entirely; in favour of a person worrying about the RNG themselves.  This is more pure and removes an unnecessary responsibility from the module.