reord install pages; per-OS instructions

doc/install/Debian.mdwn

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+If using Debian testing or unstable:
+
+	sudo apt-get install git-annex
+
+git-annex is not yet in a Debian stable release, but the source
+can be built from Debian stable. Just [[download]] it from git, and
+then in the `git-annex` directory, run `dpkg-checkbuilddeps` and install
+the necessary packages. You can also run `sudo debian/rules binary` to build
+a `git-annex.deb`.

doc/install/OSX.mdwn

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+<pre>
+sudo port install haskell-platform git-core ossp-uuid md5sha1sum
+sudo cabal update
+sudo cabal install missingh
+sudo cabal install utf8-string
+sudo port install pcre
+sudo cabal install pcre-light
+sudo cabal install quickcheck  
+
+git clone  git://git.kitenet.net/git-annex
+
+cd git-annex
+make
+sudo make install
+</pre>
+
+Originally posted by Jon at <https://gist.github.com/671785> --[[Joey]]

doc/install/Ubuntu.mdwn

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+If using Ubuntu natty or newer:
+
+	sudo apt-get install git-annex
+
+Otherwise, see [[Debian]] manual installation instructions.