Consolidate all Proxy url params to ProxyUri (issue823042)

[zh...@gmail.com]

Reviewers: shindig.remailer_gmail.com, johnfargo,

Description:
The latest rewriters refactoring introduce new ProyUri class to handle
proxy paramaters.
Some parameters like image resiaze, fallback url, mime type etc, are not
captured by it, and specifically handled by the proxy servlet.

The change here add all the params to ProxyUri, and eliminate the usage
of the original url by the proxy servlet/handler.

It also make the proxy Uri responsible for the conversion to url
parameter (it already did the parsing in the constructor)

The change is not complete, I send it out for preliminary/design review.
Update is coming soon.
TODO:
- Complete tests
- Apply same rules for concat



Please review this at http://codereview.appspot.com/823042/show

Affected files:
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/ProxyingVisitor.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/image/BasicImageRewriter.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyHandler.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/uri/ConcatUriManager.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/uri/DefaultProxyUriManager.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/uri/ProxyUriBase.java
    
java/gadgets/src/main/java/org/apache/shindig/gadgets/uri/ProxyUriManager.java
   java/gadgets/src/main/java/org/apache/shindig/gadgets/uri/UriCommon.java
    
java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/ProxyHandlerTest.java
    
java/gadgets/src/test/java/org/apache/shindig/gadgets/uri/DefaultProxyUriManagerTest.java