XML processor should use website cache

[mikey]

To improve performance of the XML/XSL view engine, the xml processor should make use of a website cache.

Refactoring #129 is a prerequisite for this.

[mikey]

With r1433 xml generators got methods to check for cache ability and cache keys.

[mikey]

To simplify implementation a huge refactoring of the memphis processor took place with r1459. Now the net::stubbles::websites::cache::stubCachingProcessor wraps the processor to be cached and contains the cache logic. Therefore the xml processor now only needs to implement the net::stubbles::websites::cache::stubCachableProcessor interface.

[mikey]

Implemented with r1467.