Breaking out the find so that it can be easily overloaded for caching

Before this commit you have to do some hacks to cache model finds easily. When using
custom find methods like `find('foo')` => `_findFoo(...)` it is common for the
method to adjust the query params.

Trying to cache a query using a hash of the query params :

	function find($type, $query) {
		$query = $this->_beforeFind($query);
		$cacheKey = $type . '_' . md5(selialize($query));

		$cache = Cache::read($cachKey, 'my_cache');
		if ($cacheKey !== false) {
			return $cache;
		}

		$results = $this->_afterFind($type, $query);
		Cache::write($cacheKey, $results, 'my_cache');

		return $results;
	}

Before this commit you either have to completely overload find and rewrite it in the AppModel or call
the before to get the modified `$query` and let cake run the before again.
adjusting the changes and adding some docs
Merge pull request #1123 from dogmatic69/model-cache

Breaking out the find so that it can be easily overloaded for caching