dist: Tweak the OSX pkg installer

This performs a few touch-ups to the OSX installer:

* A rust logo is shown during installation
* The installation happens to /usr/local by default (instead of /)
* A new welcome screen is shown that's slightly more relevant

src/etc/pkg/Distribution.xml

@@ -14,9 +14,13 @@
             <line choice="org.rust-lang.rust"/>
         </line>
     </choices-outline>
-    <choice id="default"/>
+    <choice id="default"
+            customLocation="usr/local"/>
     <choice id="org.rust-lang.rust" visible="false">
         <pkg-ref id="org.rust-lang.rust"/>
     </choice>
     <pkg-ref id="org.rust-lang.rust" version="0" onConclusion="none">rust.pkg</pkg-ref>
+    <welcome file="welcome.rtf" mime-type="text/enriched"/>
+    <background file="rust-logo.png" mime-type="image/png"
+                alignment="bottomleft"/>
 </installer-gui-script>

src/etc/pkg/welcome.rtf

@@ -0,0 +1,12 @@
+{\rtf1\ansi\ansicpg1252\cocoartf1265\cocoasubrtf190
+{\fonttbl\f0\fswiss\fcharset0 Helvetica;}
+{\colortbl;\red255\green255\blue255;}
+\margl1440\margr1440\vieww10800\viewh6300\viewkind0
+\pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\pardirnatural
+
+\f0\fs28 \cf0 \
+\
+	You will be guided through the steps necessary to\
+	install this software.\
+\
+	This will install Rust to /usr/local}