just install npm for nodejs below v0.6.3

since nodejs v0.6.3 npm is provided via nodejs installation.
so it's not required anymore to install npm separately

added tests for is_npm_provided function

:nodejs_stable_version => 'stable'

:nodejs_stable_version => 'stable' is not a valid version any more. the
version stored in facter is expected to be semver.org compatible
Merge pull request #17 from grafjo/patch-2

just install npm for nodejs below v0.6.3