[Twisted-Python] Fair warning: case-insensitive attributes

[Cory Dodt]

Since this could be both a very useful and a very harmful change, I'm 
posting a set of fairly short diffs to tm.com:
   http://twistedmatrix.com/users/moonfallen/diffs.txt
If anyone thinks this patch will slow things down (possible) or break 
stuff (unlikely; current tests pass), I want to know that.

Of course there are new tests to go with it:
   http://twistedmatrix.com/users/moonfallen/test_diffs.txt

In particular, Woven appears to be unaffected.  If I don't get any 
significant backlash over the weekend, this goes in.  This makes the 
following code possible:

 >>> d=microdom.parseString('<script LANGUAGE="JScript"></script>')
 >>> d.documentElement.setAttribute('language', 'javascript')
 >>> d.toxml()
'<?xml version="1.0"?><script language="javascript"></script>'

(Note that caseInsensitive=1 and preserveCase=0 are the defaults, and 
always have been.  They just affect attribute names now, as well as 
element names.)