Combining entropy backends

[photm5]

The problem described in vincenthz/hs-crypto-random#10 seems to be in here too. It would be nice if the entropy would be mixed from different backends, maybe using XOR for every bit.

[vincenthz]

I'm open to alternatives from/for people that want them; I'm not convinced that we need to change the current behavior to be XOR only though.

[vincenthz]

somewhat related is #13