Added support in PagerOptions for custom displaytext on next/previous links

Added support in PagerOptions for custom displaytext on next/previous links.

Instead of "<<" and ">>" you might want the words "Next" and "Previous".
Example of usage:
```
@Html.Pager(Model.PageSize, Model.PageNumber, Model.TotalItemCount).Options(o => o.AddRouteValue("categoryname", ViewBag.CategoryDisplayName)
                                                                   .SetNextPageText("Next")
                                                                   .SetPreviousPageText("Previous"))
```