Implement platform-specific dependencies
Fill `ops::registry::transmit` with the right value
Fix `sources::registry::parse_registry_dependency`
Update deps with fail => panic

Also fix a few test's assertion messages
auto merge of #779 : alexcrichton/cargo/update-deps, r=alexcrichton
Say where examples are placed after they are compiled
auto merge of #735 : tomaka/cargo/platform-specific-deps, r=alexcrichton

cc #610
auto merge of #780 : EduardoBautista/cargo/add-example-output-path-to-docs, r=alexcrichton

Solves #764.