Add pagination support for home page, sections and taxonomies

Two new configuration properties, `Paginate` (default `0`) and `PaginatePath` (default `page`) are added.

Setting `paginate` to a positive value will split the list pages for the home page, sections and taxonomies into chunks of size of the `paginate` property.

A `.Paginator` is provided to help building a pager menu.

There are two ways to configure a `.Paginator`:

1. The simplest way is just to call `.Paginator.Pages` from a template. It will contain the pages for "that page" (`.Data.Pages` will (like today) contain all the pages).
2. Select a sub-set of the pages with the available template functions and pass the slice to `.Paginate` : `{{ range (.Paginate (where .Data.Pages "Type" "post")).Pages }}`

**NOTE:** For a given Node, it's one of the options above. It's perfectly legitimate to iterate over the same pager more than once, but it's static and cannot change.

The `.Paginator` contains enough information to build a full-blown paginator interface.

The pages are built on the form (note: BLANK means no value, i.e. home page):

```
[SECTION/TAXONOMY/BLANK]/index.html
[SECTION/TAXONOMY/BLANK]/page/1/index.html => redirect to  [SECTION/TAXONOMY/BLANK]/index.html
[SECTION/TAXONOMY/BLANK]/page/2/index.html
....
```

Fixes #96

hugolib/site_url_test.go

@@ -56,6 +56,7 @@ func TestPageCount(t *testing.T) {
 	hugofs.DestinationFS = new(afero.MemMapFs)
 
 	viper.Set("uglyurls", false)
+	viper.Set("paginate", 10)
 	s := &Site{
 		Source: &source.InMemorySource{ByteSource: urlFakeSource},
 	}