Auto merge of #3319 - alexcrichton:more-info, r=brson

Emit more info on --message-format=json

This adds more output on Cargo's behalf to the output of
`--message-format=json`. Cargo will now emit a message when a crate is finished
compiling with a list of all files that were just generated along with a message
when a build script finishes executing with linked libraries and linked library
paths.

Closes #3212