Recommend Spectrum as `input type=color` polyfill

Reasons include:

* it's actively maintained
* it was good enough to get added to WebKit/Blink's dev tools
  http://bgrins.github.io/spectrum/#why-devtools
* it's MIT-licensed
* it's widely compatible (IE 6+)
* it has CI and a test suite set up
* it has an issue tracker (one of its competitors doesn't)

Close #h5bp/html5please#311.
Update input-datetime.md

the link to webshims was pointing at their old github repo. Changed it to : http://tests-anciens.ljouhet.net/webshims/