fixed issue with 'identifier list' being appended to list conversions

**Issue:**

```
$ python testParser.py 7

.
.
.
def arrays(x, y, z):
    a = [('x, y, z', 'identifier_list')]  <---- this line!
    b = a[1]

if __name__ == '__main__':
	main()
```

**Fix:** I think the issue was that _identifier_list was returning a tuple (code, type) and secondary expression was appending a string representation of this tuple instead of only appending the code part. Now:

```
$ python testParser.py 7

.
.
.
def arrays(x, y, z):
    a = [x, y, z]
    b = a[1]

if __name__ == '__main__':
	main()
```

I'm not sure if this creates more issues with the type/scope checking that Emily implemented.