Git versioning

Committed lib modules and bin scripts have the string
$VERSION = '0'

This is updated to the latest git tag and SHA in the blib version on install action, so the installed source has a version number similar to the svn one.

A new module npg_tracking::until::build.pm has the npg Module::Build subclass with the shared actions from npg_tracking and npg_common. Actions just performed by npg_tracking remain in Build.PL as a dynamic subclass of npg_tracking::until::build.