use SHA256 by default

To get old behavior, add a .gitattributes containing: * annex.backend=WORM

I feel that SHA256 is a better default for most people, as long as their
systems are fast enough that checksumming their files isn't a problem.
git-annex should default to preserving the integrity of data as well as git
does. Checksum backends also work better with editing files via
unlock/lock.

I considered just using SHA1, but since that hash is believed to be somewhat
near to being broken, and git-annex deals with large files which would be a
perfect exploit medium, I decided to go to a SHA-2 hash.

SHA512 is annoyingly long when displayed, and git-annex displays it in a
few places (and notably it is shown in ls -l), so I picked the shorter
hash. Considered SHA224 as it's even shorter, but feel it's a bit weird.

I expect git-annex will use SHA-3 at some point in the future, but
probably not soon!

Note that systems without a sha256sum (or sha256) program will fall back to
defaulting to SHA1.

doc/walkthrough/adding_files.mdwn

@@ -2,8 +2,8 @@
 	# cp /tmp/big_file .
 	# cp /tmp/debian.iso .
 	# git annex add .
-	add big_file ok
-	add debian.iso ok
+	add big_file (checksum...) ok
+	add debian.iso (checksum...) ok
 	# git commit -a -m added
 
 When you add a file to the annex and commit it, only a symlink to