install: better warning about unsupported packages

Added a warning message that is printed at the end of npm output.

Modified tree listing to mark supported/unsupported packages using
a different color.

Sample output:

![screen shot 2013-08-19 at 8 12 40](https://f.cloud.github.com/assets/1140553/984313/71cf6832-0896-11e3-93d4-0f159882d8e3.png)

//cc: @chandadharap please check the screenshot if the message and colors are ok

//cc: @piscisaureus please review the code