Allow asm printing for CALL(RIPRelativeOffset(3)) (#105)

Previously it printed:

    CALL <peachpy.x86_64.operand.RIPRelativeOffset object at 0x7f6e1e126430>

Now it prints:

    CALL rip+28

Author: certik

peachpy/x86_64/operand.py

@@ -427,6 +427,17 @@ def __add__(self, extra_offset):
     def __sub__(self, extra_offset):
         return RIPRelativeOffset(self.offset - extra_offset)
 
+    def __str__(self):
+        return self.format("peachpy")
+
+    def format(self, assembly_format):
+        if assembly_format == "gas":
+            return "%d(%%rip)" % self.offset
+        elif assembly_format == "go":
+            return "%d(IP)" % self.offset
+        else:
+            return "rip%+d" % self.offset
+
 
 def is_al(operand):
     from peachpy.x86_64.registers import GeneralPurposeRegister8, al