Issue #375397 by sun, Xano, Rob Loach: Make Node module optional.

core/includes/common.inc

core/includes/module.inc

core/includes/path.inc

core/modules/book/book.info

core/modules/comment/comment.info

core/modules/field_ui/field_ui.test

core/modules/forum/forum.info

core/modules/help/help.module

core/modules/locale/locale.install

core/modules/node/node.info

core/modules/node/node.install

core/modules/node/node.module

core/modules/overlay/overlay.module

core/modules/poll/poll.info

core/modules/rdf/tests/rdf_test.info

core/modules/shortcut/shortcut.install

core/modules/simpletest/tests/database_test.test

core/modules/simpletest/tests/theme.test

core/modules/simpletest/tests/theme_test.module

core/modules/system/system.admin.inc