Calculate response Hash value (issue391042)

[zhoresh]

Reviewers: shindig.remailer_gmail.com, johnfargo,

Description:
This feature will be used by the Uri rewritters to create versions for
resources.
The request pipeline calculates the md5 for each response and store it
in the HttpResponse metadata map.

The added class HttpResponseMetadataHelper will support updating the
metadata for other values, such as image dimensions to supoprt img link
rewritter (will be done if next CL).



Please review this at http://codereview.appspot.com/391042/show

Affected files:
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/http/DefaultRequestPipeline.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/http/HttpResponseMetadataHelper.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/http/DefaultInvalidationServiceTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/http/DefaultRequestPipelineTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/http/HttpResponseMetadataHelperTest.java