Fix --extern for doc tests for the local crate.

Closes #340
Verbosely say what's happening during doc tests

Or at least verbosely say when -v is provided.
Make sure version env vars are always present

They were accidentally left out of the `rustdoc --test` stage which caused
crates not to be able to be compiled.
auto merge of #341 : alexcrichton/cargo/doc-test-tweaks, r=wycats
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