Support to date, datetime and time inputs.

I changed `DateTimeInput` to support the new HTML5 form helpers that
comes with Rails 4 (`date_input`, `datetime_input` and `time_input`)
instead of rendering datetime selects:

For example, this input:
```erb
<%= f.input :born_on, as: :date %>
```
Will render:
```html
<input id="user_born_on" name="user[born_on]" type="date" />
```

Datetime selects are still present, but only when HTML5 compatibility
is disabled. It's also possible to render selects even with HTML5 on,
by setting the `html5` option to false.

If we wanted to use an old date select on the example above, we should do:
```erb
<%= f.input :born_on, as: :date, html5: false %>
```

Closes #844.
Assert `input` tags instead of `select` for datetime attributes.
Do not assert `required` HTML5 attribute when HTML5 compatibility is disabled.
Add a CHANGELOG entry regarding datetime inputs.
Remove unecessary `enable` call.
Merge branch 'master' into vs-html5-datetime-inputs

Conflicts:
	CHANGELOG.md
Use classic datetime selects unless HTML5 is explicitly enabled.
Just `input_options[:html5]` is enough.
Fix mapping table regarding datetime columns.
Merge branch 'master' into vs-html5-datetime-inputs

Conflicts:
	CHANGELOG.md
Merge pull request #935 from plataformatec/vs-html5-datetime-inputs

Support to date, datetime and time inputs.