Implement absolute ROP loading given a static base address

Implement absolute ROP addressing, and recursive string/struct loading based off of absolute addresses.

### Script

```python
from pwn import *

bash = ELF('/vortex/vortex13')
libs = gdb.find_module_addresses(bash.path)
rop  = ROP(libs+[bash])


rop.migrate(0x1000000)
print '\n'.join(rop.dump())
print '\n'*3


rop.flush()
sz   = "Hello, Gallopsled\n"

rop.set_address(0x1000000)
rop.mmap("/tmp/baz","w+")
rop.write(1, sz, len(sz))

print '\n'.join(rop.dump())
print '\n'*3
print hexdump(str(rop), begin=rop.address)
```

### Output

```
[+] Started program 'gdb'
[*] Loading gadgets for '/lib/i386-linux-gnu/libc.so.6' @ 0xf7557000
[+] Binary loaded
[+] Loading gadgets, please wait...
[+] Gadgets loaded !
[*] Loading gadgets for '/lib/ld-linux.so.2' @ 0xf7727000
[+] Binary loaded
[+] Loading gadgets, please wait...
[+] Gadgets loaded !
[*] Found gadgets for '/vortex/vortex13' in cache '/tmp/user/pwntools-rop-cache/vortex13-585d3aab6cc29a38037fd8f866452fb5-0x8048000'
0000: f87872f7       0xf77278f8 pop ebp; ret
0004: 00000001        0x1000000 
0008: 5ab65ff7       0xf75fb65a leave; ret




1000000: c0ea63f7       0xf763eac0 mmap
1000004: 557e72f7       0xf7727e55 pop esi; pop edi; ret
1000008: 24000001        0x1000024  <========= +24
100000c: 2c000001        0x100002c  <========= +2C
1000010: 702763f7       0xf7632770 write
1000014: efbeadde       0xdeadbeef 
1000018: 01000000              0x1 
100001c: 30000001        0x1000030  <========= +30
1000020: 12000000             0x12 
1000024: 2f746d70       0x706d742f 
1000028: 2f62617a       0x7a61622f 
100002c: 772b0000           0x2b77 
1000030: 48656c6c       0x6c6c6548 
1000034: 6f2c2047       0x47202c6f 
1000038: 616c6c6f       0x6f6c6c61 
100003c: 70736c65       0x656c7370 
1000040: 640a0000            0xa64 




01000000  c0 ea 63 f7 55 7e 72 f7   24 00 00 01 2c 00 00 01  |..c.U~r.$...,...|
01000010  70 27 63 f7 ef be ad de   01 00 00 00 30 00 00 01  |p'c.........0...|
01000020  12 00 00 00 2f 74 6d 70   2f 62 61 7a 77 2b 00 00  |..../tmp/bazw+..|
01000030  48 65 6c 6c 6f 2c 20 47   61 6c 6c 6f 70 73 6c 65  |Hello,.Gallopsle|
01000040  64 0a 00 00                                        |d...|
00000044
```