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/using_ssh_remotes.mdwn

@@ -13,7 +13,7 @@ Now you can get files and they will be transferred (using `rsync` via `ssh`):
 
 	# git annex get my_cool_big_file
 	get my_cool_big_file (getting UUID for origin...) (from origin...)
-	WORM-s2159-m1285650548--my_cool_big_file       100% 2159     2.1KB/s   00:00
+	SHA256-s86050597--6ae2688bc533437766a48aa19f2c06be14d1bab9c70b468af445d4f07b65f41e  100% 2159     2.1KB/s   00:00
 	ok
 
 When you drop files, git-annex will ssh over to the remote and make