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
auto merge of #13185 : alexcrichton/rust/osx-pkg, r=brson

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