[FormsySelect] Add support for composable SelectField

[ryanblakeley]

fix #40
Fixes issues with DropDownMenu where it wants MenuItems passed as children (composability).

handleChange reflects updates to the material-ui API. It takes (event, index, value) so we can directly access the value, rather than going through event.target.value.

It adds a state variable hasChanged for deciding whether to pass the initial props.value (there is no unique initialValue prop) or the formsy mixin method this.getValue().

[mbrookes]

Thank you very much @rblakeley, that's great! 👍 Please could you squash / fixup, then I can merge.

You can leave out the prepubs - I'll push that as part of the version bump.

[ryanblakeley]

No worries. I was running into the same issue in my app.

It's squashed now.