dns: include host name in error message if available

This makes errors more readable and similar to FS errors, which also
include file name.

Reviewed-By: Fedor Indutny <fedor@indutny.com>
lib, src: add vm.runInDebugContext()

Compiles and executes source code in V8's debugger context.  Provides
a programmatic way to get access to the debug object by executing:

    var Debug = vm.runInDebugContext('Debug');

Fixes #7886.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
node: Now working on v0.13.0
adding instructions about fixing dynamic module loading