auto merge of #612 : alexcrichton/cargo/nocapture, r=wycats

auto merge of #612 : alexcrichton/cargo/nocapture, r=wycats

There are some competing concerns when it comes to the output of compiling
dependencies:

* Not capturing anything leads to getting drowned in unrelated output
* Capturing requires coloration be compromised because of the way windows
  terminal colors are implemented.
* Path dependencies are often developed in tandem with the rest of a package,
  and capturing their output is not always desired.

To address these concerns, cargo previously captured output of dependent
compilations and then re-printed it to the screen if an error occurred. This
patch modifies the behavior to as follows:

* No output is captured. This preserves any coloration rustc provides.
* All dependencies are compiled with `-Awarnings`. This should suppress any
  extraneous output from the compiler and it is considered a bug otherwise if
  the compiler prints a warnings when `-Awarnings` is specified.
* All *path* dependencies (`path="..."`, overrides, etc) are *not* compiled with
  `-Awarnings`. The reason for this is that you are always in control of these
  packages and probably want to see warnings anyway.

Closes #490
Closes #496