Make ServiceManager dependency optional in Feed component

[weierophinney]

This PR makes the ServiceManager dependency in the Feed component optional.

• Creates Zend\Feed\Reader\ExtensionManagerInterface, defining required capabilities for an extension manager; ExtensionManager implements this interface now.
• Creates Zend\Feed\Writer\ExtensionManagerInterface, defining required capabilities for an extension manager; ExtensionManager implements this interface now.

If desired, you can substitute your own implementations using:

Reader::setExtensionManager($customImplementation);
Writer::setExtensionManager($customImplementation);

[weierophinney]

Ping @padraic

[weierophinney]

@padraic There's one problem: 5.3.3: https://travis-ci.org/zendframework/zf2/jobs/6834575#L347

Basically, a change was made somewhere in the 5.3 series, perhaps as early as 5.3.4, which fixed this particular behavior, allowing a class to implement multiple interfaces with the same signatures.

I'm not sure how to proceed at this point, other than to potentially use some sort of polyfill to have a marker interface in 5.3.3 and the real interface in later versions. Thoughts?

[padraic]

@weierophinney Looks good, through I don't know what to do about the 5.3.3 problem.

[weierophinney]

@padraic Already resolved -- I used a Decorator to make it work.