TextWidget now reuses the last submitted value.

Currently, TextWidget won't use the value from a previous submission of the form.  I'm not 100% comfortable with my solution, as the iterating to display values for a field that should likely never be an actual array seems a little hokey.  But, that's the data type getting passed to the widget, so I stuck with it.

Here was my example form that I triggered this with:

	loginForm := gforms.DefineForm(gforms.NewFields(
		gforms.NewTextField("Username",
			gforms.Validators{
				gforms.Required(),
			},
			gforms.NewTextWidget(map[string]string{"class": "form-control"}),
		),
		gforms.NewTextField("Password",
			gforms.Validators{
				gforms.Required(),
				gforms.MinLengthValidator(1),
			},
			gforms.NewTextWidget(map[string]string{"class": "form-control"}),
		),
	))