Add launchctl agent plist for OSX

I found this useful to have, and figured it might save somebody else a
few minutes of re-inventing the wheel.

misc/launchd/README.md

+# ipfs launchd agent
+
+A bare-bones launchd agent file for ipfs. To have launchd automatically run the ipfs daemon for you, do the following:
+
+    mkdir -p ~/Library/LaunchAgents
+    cp misc/launchctl/io.ipfs.ipfs-daemon.plist ~/Library/LaunchAgents/
+    launchctl load ~/Library/LaunchAgents/io.ipfs.ipfs-daemon.plist
+
+Note that the `ipfs` binary must be on the *system* PATH for this to work. Adding a symlink in /usr/bin works well enough for me.

misc/launchd/io.ipfs.ipfs-daemon.plist

+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+  <dict>
+    <key>KeepAlive</key>
+    <true/>
+    <key>Label</key>
+    <string>io.ipfs.ipfs-daemon</string>
+    <key>ProgramArguments</key>
+    <array>
+      <string>ipfs</string>
+      <string>daemon</string>
+    </array>
+    <key>RunAtLoad</key>
+    <true/>
+    <key>StandardErrorPath</key>
+    <string>/usr/local/var/log/ipfs.err</string>
+    <key>StandardOutPath</key>
+    <string>/usr/local/var/log/ipfs.log</string>
+  </dict>
+</plist>
+