Fixes #150: Native date time widgets.

[n1k0]

WiP, refs #150 /cc @kaihendry

This adds new native date and datetime widgets which are now used by default, and demotes previous default select-based date widgets as alternative ones.

Todo

• Update docs
• Write tests (hard as jsdom doesn't have a clue what these inputs are)

[n1k0]

We should probably keep the select-based date widgets as default ones, as support of native ones isn't great across platforms just yet.

Edit: after discussing about that with @leplatrem, we decided to keep it this way.

[leplatrem]

nit: May not work on some browsers could be enough ?

[leplatrem]

FYI it works like a charm on Firefox mobile:

[almet]

after discussing about that with @leplatrem, we decided to keep it this way.

Can we know what the turning point was?

As the default HTML5 date input doesn't work on almost all non-mobile platforms. I would suggest having something that works for all platforms by default instead.

[n1k0]

Can we know what the turning point was?

Pushing and promoting Web standards.

[n1k0]

Note: For Firefox, bug 825294 should land anytime soon. MSDN strangely declares it supported, so maybe it's just matter of time for it to land on a stable version?

[almet]

Mouarf, pushing and promoting web standards is great, but we should make our library users aware that this will not work for most of their users, and probably propose an alternative solution.

[n1k0]

we should make our library users aware that this will not work for most of their users, and probably propose an alternative solution

That's probably why I've added a specific section in the documentation along providing alternative widgets compatible with those other platforms.

[almet]

Perfect! :)

[almet]

I believe imgur isn't a good place to host these images, as the pictures could disapear any time. Should we host them in this very repository instead?

[n1k0]

There's no easy way to make github serving images hosted in the repo itself. Also, if these are ever gone it's no big deal to redo some captures.

[almet]

I believe it's possible to do it like that:

![alt tag](https://raw.githubusercontent.com/mozilla-services/react-jsonschema-form/master/path/to/img.png)

[n1k0]

I'm gonna land this with r=me.