server error when uploading packages with no dependencies

[rbuckheit]

The server errors out when a package with no dependencies is uploaded. This is because node_modules does not get created in the data directory, and then we try to tar an archive that isn't there.

repro:

1. create a package.json with no dependencies or devDependencies
2. submit the package via an npmserve install

expected: return a tarball which decompresses, yielding an empty node_modules
actual: server 500 error, server responds with an error.

[rbuckheit]

fixed in #17