'meteor create' should output writable files

(this is a followup to the commit where we made files.cp_r preserve
exectubility, which was messing up the +x bit on the meteor-tool
package)
allow saveToPath twice
compare built tarball hash instead of buildid

buildids just change too damn much.  oh no, you changed a comment in
random which is transitively used by templating which processes .html, I
guess you need to bump all the versions of all packages with .html?
that sucks.  how about only changing it if the EFFECT of the build is
changed?

this does have some subtleties around platform-specific versions but we
think we have a good compromise
use catalog better