Complicate entropy rng 0.5

[pitdicker]

#508 Rebased on the 0.5 branch.

This is (in my opinion) the proper fix for #503: make OsRng unavailable if the platform is not supported.

This also makes JitterRng::new unavailable on Wasm.

EntropyRng already had tricky logic. Making it work nicely when OsRng and/or JitterRng are not available at compile time complicates it further. And I didn't want to think how it was going to look when we want to add some configurable third entropy source.

I have tried to rewrite EntropyRng to have hopefully simpler logic. Because we need wrappers for OsRng and JitterRng to make it compile on all platforms, I went all the way and added a little trait to ease implementation.

[pitdicker]

I wonder why Travis doesn't run?

[pitdicker]

I am going to merge, so I can then do the merge of 0.5 in master, but will keep an eye on the Travis build that should start up for the 0.5 branch.