Simplify header class hierarchy:

* use static valueOf factory methods for parsing headers from messages and strings
* use normal constructors for creating new headers programmatically
* remove equals/hashCode implementations since they seem redundant
* move Header interface to top-level package
* TODO: add setters for all headers
* TODO: change Headers class from MultiMap<String,String> to MultiMap<String,Header> in order to facilitate updates to headers, especially cookies.

Simplify IO package:

* hide most implementation classes
* expose everything via static methods in new IO utility class
* TemporaryStorage class is still required in openig-core in order to play nicely with the config framework.



git-svn-id: https://svn.forgerock.org/openig/branches/http-framework@533 dbb9e58e-28e6-4ce0-90e8-f11d9605b710