fix, and shrink, the gopher icon, txt extensions
for license and readme files, minor format changes
in the wxs file
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5683054
Embedded Go images in the installer dialog and
banner boxes, Go tree cloning uses version info
from the Go tool (readme updated), the installer
allows up/down-grading and reparing of the version
that's installed, added two registry values.
R=golang-dev, bradfitz, bradfitz
CC=golang-dev
https://golang.org/cl/5683048
