Refactor cargo_test into an ops module

The logic for doc tests will get a little complex, so this is moved to aseparate
module instead of inside the executable.
Implement doc tests

Whenever `cargo test` is run and a testable library target is available, the doc
tests will be run. This can be opted out of with `test = false` as usual.

This is currently not super useful due to rust-lang/rust#16157, but I expect
that to be merged soon. In the meantime examples will need to `extern crate foo`
explicitly.
auto merge of #302 : alexcrichton/cargo/doc-test, r=wycats

Whenever `cargo test` is run and a testable library target is available, the doc
tests will be run. This can be opted out of with `test = false` as usual.

This is currently not super useful due to rust-lang/rust#16157, but I expect
that to be merged soon. In the meantime examples will need to `extern crate foo`
explicitly.