[WIP/RFC] Refactored CacheInvalidator into descreet units

- Renamed and repurposed CacheInvalidator => CacheManager
- CacheManager now delegates to handlers
- Added handlers for each use case

Usage:

````php
$cache = new Varnish();
$pathHandler = new PathHandler($cache);
$tagsHandler = new TagsHandler($cache);

$cacheManager = new CacheManager(array(
    'path' => $pathHandler,
    'tags' => $pathHandler,
));

// invalidate by path
$cacheManager->invalidate('path', '/path/to');

// invalidate by path with headers
$cacheManager->invalidate('path', '/path/to', array(
    'headers' => array(
        'X-Foo' => 'bar'
    )
);

// invalidate tags
$cacheManager->invalidate('tags', array('foo', 'bar'));

// refresh a path
$cacheManager->refresh('path', '/path/to');

// flush -- will flush all the cache implementations used
// by the handlers (once)
$cacheManager->flush();
````

Each handler has its own cache proxy instance. This would allow different cache
implementations to be used simultaneously if required.

NOTE: Tests are currently missing.

As noted in [the docs](http://foshttpcache.readthedocs.org/en/latest/cache-invalidator.html), we prefer the fluent interface so one can:
* refresh multiple paths in one go
* stick `flush()` right after the invalidation command; I guess this one is not relevant anymore in the handlers?