Added a Today button for Date and Datetime fields.

As mentioned in this issue:
https://github.com/JedWatson/keystone/issues/125

I did just see, @JedWatson that you commented on it doing it after react has landed. If it is not too far of, then nevermind in merging this. Just felt like taking a stab at this and turned out not to be too much effort,
Not sure why the two lines above were removed. These were added in #499 to fix issue #498.