Implement list pop method

Attribute errors occured using list.
​

**Python (expected)**
```
>>> a.pop()
4
>>>a
[1,2,3]
>>>a.pop(1)
2
>>>a
[1,3]
```

**gpython (Actual)**
```
>>> a = [1,2,3,4]
>>> a.pop()
Traceback (most recent call last):
File "", line 1, in
FIXME line of source goes here
AttributeError: "'list' has no attribute 'pop'"
```