Truncated; .Site.Params; First function

* Add `.Truncated` bool to each page; will be set true if the
  `.Summary` is truncated and it's worth showing a "more" link of some
  kind.
* Add `Params` to the site config, defining `.Site.Params` accessible
  to each page; this lets the site maintainer associate arbitrary data
  with names, on a site-wide basis.
* Provide a `First` function to templates:
  * Use-case: `{{range First 5 .Site.Recent}}` or anything else which
    is a simple iterable provided by hugolib
* Tests by me for `.Truncated` and `First`

Also @noahcampbell contributed towards this:

* Add UnitTest for `.Site.Params`:
> Digging into this test case a bit more, I'm realizing that we need
> to create a param test case to ensure that for each type we render
> (page, index, homepage, rss, etc.) that the proper fields are
> represented.  This will help us refactor without fear in the
> future.

Sample config.yaml:

```yaml
title: "Test site"
params:
  Subtitle: "More tests always good"
  AuthorName: "John Doe"
  SidebarRecentLimit: 5
```

Signed-off-by: Noah Campbell <noahcampbell@gmail.com>

hugolib/config.go

@@ -33,6 +33,7 @@ type Config struct {
 	Title                                      string
 	Indexes                                    map[string]string // singular, plural
 	ProcessFilters                             map[string][]string
+	Params                                     map[string]interface{}
 	BuildDrafts, UglyUrls, Verbose             bool
 }