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 modules intention (BIP39), and therefore I actually suggest an alternative PR where `generateMnemonic` is removed entirely; in favour of leaving the developer to use an RNG themselves.
This is more pure and removes an unnecessary responsibility from the module.