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. 
The rng passed in must implement the same interface. In this case, `randomBuffer` is neither the browser interface nor the node interface =/