win: make nodejsvars.bat detect whether npm is available
win/msi: define features before directories/components
win/msi: don't install pdb file

It's only available in debug builds, and we're not providing
installer packages for debug builds anyway.
win/msi: don't create empty npm folder in %appdata%

Npm creates this folder when it's needed. Creating it in the installer
violates the per-user / per-machine scope separation.
win/msi: use consistent registry key paths
win/msi: make npm a feature separate from the runtime
win/msi: separate features for ETW and perfctr support
win/msi: make 'add to path' a separate feature
win/msi: refactor 'node.js runtime' feature

* This feature now includes the start menu items.
* 'nodejsvars.bat' was renamed to 'nodevars.bat'.
* Improved feature description.
win/msi: clean up the 'documentation shortcuts' feature
win/msi: update 'install finished' message
win/msi: don't include architecture in start menu group name
win/msi: miscellaneous style cleanups
win/msi: enable modify and repair