optimize: A new subcommand that either gets or drops file content as needed to work toward meeting the configured numcopies setting.

This is currently rather simplistic, though still useful.
In the future, it could become smarter about what content is stored where,
etc.

doc/git-annex.mdwn

@@ -157,6 +157,11 @@ Many git-annex commands will stage changes for later `git commit` by you.
 
   To avoid expensive checksum calculations, specify --fast
 
+* optimize [path ...]
+  
+  Either gets or drops file content, as needed, to work toward meeting the
+  configured numcopies setting.
+
 * unused
 
   Checks the annex for data that does not correspond to any files present

doc/walkthrough.mdwn

@@ -18,5 +18,6 @@ A walkthrough of the basic features of git-annex.
 	fsck:_verifying_your_data
 	fsck:_when_things_go_wrong
 	backups
+	optimizing_repositories
 	more
 """]]

doc/walkthrough/optimizing_repositories.mdwn

@@ -0,0 +1,13 @@
+Once you have multiple repositories, and have perhaps configured numcopies,
+any given file can have many more copies than is needed, or perhaps fewer
+than you would like. Fsck can detect the latter problem, but there's another
+command that can help deal with both problems.
+
+The optimize subcommand either gets or drops file content, as needed,
+to work toward meeting the configured numcopies setting.
+
+	# git annex optimize
+	get my_cool_big_file (from laptop...) ok
+	drop other_file ok
+	# git annex optimize --numcopies=2
+	get other_file ok