Ticket 16304 -- Add HTML5 placeholder attribute support to form fields

As discussed in [#16304](https://code.djangoproject.com/ticket/16304), the addition of placeholders to input boxes should be easier than this:

    comment = forms.CharField(max_length=200, widget=forms.TextInput({ "placeholder": "Text!"}))

With this change, it will be something like this:

    comment = forms.CharField(max_length=200, placeholder="Text!")

Tests done and docs updated.