Nugetgallery: Allow package replacement via the Push API

[half-ogre]

I don't know if the current site supports this, but we should allow the push API to replace an existing package version. Are there any rules we want to enforce for update?

[haacked]

For now, we'll close this because we don't allow package deletes. The workaround is to upload a newer version package and unlist the old one.

[steveoh]

i would like to be able to replace a package with the same version # when I screw up the package without having to bump the version number. I like to keep the version numbers in sync with the packages code and do not want to give a false impression that the packaged code has gained a version.