Allow uploading of multiple archive files and explain about multiple architectures

Many commercial devs have expressed interest in uploading multiple archives, such as a separate build for 32 bit and 64 bit. Currently they work around this by either submitting two apps, one for each version, or by submitting a giant tarball containing both arch versions. Sometimes the latter includes the different arch files all mixed together and a script that dynamically picks which one to launch.

Neither of these are ideal, as a proper binary package will only have files for the architecture it's built for -- the packager needs to manually piece out the architecture-dependent files and remove them from the install for different architectures. This isn't just for policy reasons - dh_shlibdeps, for instance, will fail if it finds a foreign architecture file.

The solution here is to allow the uploading of multiple files (which will be needed for other reasons, eg bug #902983), and provide some sort of minimal explanation about how ubuntu packages work on different architectures. The following things are probably nonintuitive to most app developers:
 - if you supply 32-bit only, you will get a 32 bit package and the exact same package will be installed on 32/64 bit, with dependencies in turn pulled in via multiarch
 - if you supply both 32 and 64 bit, a person will get a different package depending on which arch of Ubuntu they're using, but both of these are built from the same source package and have the same name/page in software center