Add support for generating an Eclipse .classpath file

This adds support for generating a .classpath file that can be copied
to an Eclipse project for improved symbol navigation.

BUG=gyp:472
Review URL: https://codereview.chromium.org/774693004/

Patch from Kevin McCormick <mckev@amazon.com>!


git-svn-id: http://gyp.googlecode.com/svn/trunk@2015 78cadc50-ecff-11dd-a971-7dbc132099af