Implement manifest profiles

For documentation, see the included documentation in the commit.
Re-enable passing -g to rustc

Now that there is a way to disable debuginfo for a build, we can go back to
passing it by default. Any bugs in debuginfo will get weeded out by specifying
`debug = false` in the profile.
auto merge of #507 : alexcrichton/cargo/profiles, r=brson

For documentation, see the included documentation in the commit.