Added a get($key) function

Looking for a PHP dictionary implementation, this is the first one I looked at. It is small, and works. I like that.

I have added a get($key) function, defined in the interface and implemented in the class. I may add more functionality to make it more like the C# and C++ STL equivalents as that is what I am used to.

In IDictionary:
public function get( $key );

In Dictionary:
public function get( $key )
{
if ( !self::exists($key) )
throw new Exception("The given key does not exist");

return $this->items[$key];
}