Allow documenting binary targets.

This removes the check in the compilation phase, but adds an error if you're
documenting a library and a binary with the same name (as the rustdoc output
would conflict).
auto merge of #320 : alexcrichton/cargo/fix-doc-bins, r=wycats

This removes the check in the compilation phase, but adds an error if you're
documenting a library and a binary with the same name (as the rustdoc output
would conflict).

Closes #318