Recursively merge dicts.
    
    recursively merges dict's. not just simple a['key'] = b['key'], if
    both a and bhave a key who's value is a dict then dict_merge is called
    on both values and the result stored in the returned dictionary.
    
    @see http://www.xormedia.com/recursively-merge-dictionaries-in-python/

def dict_merge(a, b):
    """
    Recursively merge dicts.
    
    recursively merges dict's. not just simple a['key'] = b['key'], if
    both a and bhave a key who's value is a dict then dict_merge is called
    on both values and the result stored in the returned dictionary.
    
    @see http://www.xormedia.com/recursively-merge-dictionaries-in-python/
    """
    if not isinstance(b, dict):
        return b
    result = deepcopy(a)
    for k, v in b.iteritems():
        if k in result and isinstance(result[k], dict):
                result[k] = dict_merge(result[k], v)
        else:
            result[k] = deepcopy(v)
    return result