HER-1010 crawl favicon.ico

* ExtractorHTTP.java
    consider /favicon.ico 'discovered' from every other HTTP fetch
* Extractor.java
    utility method addOutlink()

modules/src/main/java/org/archive/modules/extractor/Extractor.java

@@ -125,6 +125,24 @@ public abstract class Extractor extends Processor {
      */
     protected abstract void extract(CrawlURI uri);
 
+
+    /**
+     * Create and add a 'Link' to the CrawlURI with given URI/context/hop-type
+     * @param curi
+     * @param uri
+     * @param context
+     * @param hop
+     */
+    protected void addOutlink(CrawlURI curi, String uri, LinkContext context,
+            Hop hop) {
+        try {
+            UURI dest = UURIFactory.getInstance(curi.getUURI(), uri);
+            Link link = new Link(curi.getUURI(), dest, context, hop);
+            curi.getOutLinks().add(link);
+        } catch (URIException e) {
+            logUriError(e, curi.getUURI(), uri);
+        }
+    }
     
     public void logUriError(URIException e, UURI uuri, 
             CharSequence l) {

modules/src/main/java/org/archive/modules/extractor/ExtractorHTTP.java

@@ -52,8 +52,12 @@ public class ExtractorHTTP extends Extractor {
     @Override
     protected void extract(CrawlURI curi) {
         HttpMethod method = curi.getHttpMethod();
+        // discover headers if present
         addHeaderLink(curi, method.getResponseHeader("Location"));
         addHeaderLink(curi, method.getResponseHeader("Content-Location"));
+        
+        // try /favicon.ico for every HTTP(S) URI
+        addOutlink(curi, "/favicon.ico", LinkContext.EMBED_MISC, Hop.EMBED);
     }
 
     protected void addHeaderLink(CrawlURI curi, Header loc) {