Add --vma-offset to asm.disasm() and ELF.disasm() helper to automate

Just a helper to get correct offsets into the disassembly, and to disassembly from virtual addresses in `ELF`s.

```
>>> from pwn import *                                                                                                                                    
>>> bash = ELF('/bin/bash')                                                                                                                              
>>> print bash.disasm( bash.symbols['_start'], 5)
  4201db:       31 ed                   xor    ebp,ebp
  4201dd:       49 89 d1                mov    r9,rdx
```