HTML helpers enum support (2 of 4): Add EnumHelper.cs
- Add new EnumHelper class providing IsValidForEnumHelper() and GetSelectList() overloads

For example in an Enum.cshtml template file:
    if (EnumHelpers.IsValidForEnumHelper(ViewData.ModelMetadata))
    {
        IList<SelectListItem> selectList = EnumHelpers.GetSelectList(ViewData.ModelMetadata, Model);
        @* Modify select list as desired *@
        @Html.DropDownListFor(model => model, selectList)
    }
    else
    {
        @Html.TextBoxFor(model => model)
    }
or
    if (EnumHelpers.IsValidForEnumHelper(ViewData.ModelMetadata))
    {
        @Html.EnumDropDownListFor(model => model)
    }
    else
    {
        @Html.TextBoxFor(model => model)
    }

Html.EnumDropDownListFor() is not part of this commit but is coming soon.