Build examples into `target/examples`

This ends up killing two birds with one stone! The rationale behind this is that
the example and bin namespaces are not the same, and we don't mix metadata into
either filename, so the outputs need to be in different locations.

Closes #193
Closes #751
auto merge of #758 : alexcrichton/cargo/issue-751, r=brson

This ends up killing two birds with one stone! The rationale behind this is that
the example and bin namespaces are not the same, and we don't mix metadata into
either filename, so the outputs need to be in different locations.

Closes #193
Closes #751