Proposed new version

This adds an options object so that you can set Math.random like this:

<pre>
seedrandom('hello.', {global: true});
rng = seedrandom('hello.', {entropy: true});
rng = Math.seedrandom('hello.', {global:false});
</pre>

Options include:

<pre>
global -    true mutates Math.random and returns seed.
            false doesn't touch Math.random and returns PRNG.
            absent mimics old behavior (Math.seedrandom:true, module:false).
entropy -   true mixes entropy.
            false or absent doesn't mix entropy.
pass -      fn supplies a callback argument. calls
            fn(prng, seed, global)
</pre>

The old calling conventions still work: you can pass a boolean
as the second argument to be treated as "entropy: bool"; and
you can pass the callback as the third arugment.

This version also adds and updates tests.