Issue #57: A multi-platform feature to embed node.js

Instead of having multiple platform-specific feature, this
now provide a single feature with platform-specific bundles
that will be installed depending on the environment.

The main tricks were:
* Set up environments in target-platform-configuration in root pom
* Setting fragment=true in feature.xml, even for bundles.
This was inspired by looking at the multiplatform.Reactor
integration-test of Tycho.
correct name should be tern.eclipse.ide.server.nodejs.embed.win32.win32.x86_64 to comply with other names ${os}.${ws}.${arch}.