Auto merge of #3301 - integer32llc:categories, r=alexcrichton

Upload categories specified in the manifest

This adds support for uploading categories to crates.io, if they are specified in the manifest.

This goes with rust-lang/crates.io#473. It should be fine to merge this PR either before or after that one; crates.io master doesn't care if the categories are in the metadata or not. With that PR, I was able to use this patch with cargo to add categories to a crate!

diff --cc src/cargo/core/manifest.rs
Simple merge

diff --cc src/cargo/ops/registry.rs
Simple merge

diff --cc src/cargo/util/toml.rs
Simple merge

diff --cc src/doc/manifest.md
Simple merge