Externalize the writing of content to a target

Introducing the target module in hugo.  This provides the simple
interface for writing content given a label (filename) and a io.Reader
containing the content to be written.

If site.Target is not set, it defaults back to the original behavior of
writing to file system.

In hugolib/site_url_test.go I have an InMemoryTarget for testing
purposes and use it to see if the final output of a render matches.

hugolib/site_url_test.go

@@ -0,0 +1,69 @@
+package hugolib
+
+import (
+	"bytes"
+	"io"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+const SLUG_DOC_1 = "---\ntitle: slug doc 1\nslug: slug-doc-1\n---\nslug doc 1 content"
+const SLUG_DOC_2 = "---\ntitle: slug doc 2\nslug: slug-doc-2\n---\nslug doc 2 content"
+
+const INDEX_TEMPLATE = "{{ range .Data.Pages }}.{{ end }}"
+
+func must(err error) {
+	if err != nil {
+		panic(err)
+	}
+}
+
+func mustReturn(ret *Page, err error) *Page {
+	if err != nil {
+		panic(err)
+	}
+	return ret
+}
+
+type InMemoryTarget struct {
+	files map[string][]byte
+}
+
+func (t *InMemoryTarget) Publish(label string, reader io.Reader) (err error) {
+	if t.files == nil {
+		t.files = make(map[string][]byte)
+	}
+	bytes := new(bytes.Buffer)
+	bytes.ReadFrom(reader)
+	t.files[label] = bytes.Bytes()
+	return
+}
+
+func TestPageCount(t *testing.T) {
+	target := new(InMemoryTarget)
+	s := &Site{Target: target}
+	s.prepTemplates()
+	must(s.addTemplate("indexes/blue.html", INDEX_TEMPLATE))
+	s.Files = append(s.Files, "blue/doc1.md")
+	s.Files = append(s.Files, "blue/doc2.md")
+	s.Pages = append(s.Pages, mustReturn(ReadFrom(strings.NewReader(SLUG_DOC_1), filepath.FromSlash("content/blue/doc1.md"))))
+	s.Pages = append(s.Pages, mustReturn(ReadFrom(strings.NewReader(SLUG_DOC_2), filepath.FromSlash("content/blue/doc2.md"))))
+
+	if err := s.BuildSiteMeta(); err != nil {
+		t.Errorf("Unable to build site metadata: %s", err)
+	}
+
+	if err := s.RenderLists(); err != nil {
+		t.Errorf("Unable to render site lists: %s", err)
+	}
+
+	blueIndex := target.files["blue/index.html"]
+	if blueIndex == nil {
+		t.Errorf("No indexed rendered. %v", target.files)
+	}
+
+	if len(blueIndex) != 2 {
+		t.Errorf("Number of pages does not equal 2, got %d. %q", len(blueIndex), blueIndex)
+	}
+}