Make flux key configurable for FluxComponent

FluxComponent and fluxMixin use the key `flux` for passing the Flux instance via props and context. This is a natural default, but it should be configurable to enable multiple Flux instances to be used without clashing — for example, if a Flummox instance is used locally by a component, or as part of a library.

API would look something like this:

``` js

// this.props.flux === fluxA && this.props.flummox === fluxB

```

(Props passing is nice, but this is more important for preventing context clashes. In this example, descendants of InnerComponent have context `flux` and `flummox`)

The API for fluxMixin would be a bit trickier. The current args for `fluxMixin()` are the same as the args for `connectToStores()`, so it would be weird to add another arg. For this reason, I'm thinking this feature should be exclusive to FluxComponent. Also, I don't think people should use fluxMixin anyway... it only exists in the first place because I know some people are stubborn and will insist on the mixin form :)