Made tools/pretend-userbuild.sh more useful

Now grabs a copy of ONL from git automatically, suitable for calling
via:

$CURL https://raw.githubusercontent.com/opennetworklinux/ONL/master/tools/pretend-userbuild.sh | bash | tee build.out

Will be part of the nightly automated build process.
Merge pull request #35 from capveg/autobuild

Made tools/pretend-userbuild.sh more useful