Add plt.XXXX and got.XXXX symbols so that rop.dump() output is better

Specifically this is to enhance `ROP.dump`.  `got.read` is not printed in the example below without this.

```
>>> rop1 = ROP(rex)
>>> rop1.call(rex.plt['write'], [1, rex.got['read'], 4])
>>> rop1.call(0x80483F4)
>>> print '\n'.join(rop1.dump())
    0x0000:        0x80482f4 (write)
    0x0004:        0x80483f4
    0x0008:              0x1
    0x000c:        0x804961c (got.read)
    0x0010:              0x4
```