Add work-in-progress for npm support
- use flags, not npmrc. This let's me skip the weird HOME hackery
  I was doing before.
- now should abort if 'npm ls' fails.
- make 'version' public (fixes tests)
- 0.4.33 with npm support!
Remove unused accessor/ivar
- don't use @excludes; just make it a local variable since the
--exclude flag populates the attributes[:excludes] setting.
- fix_dependency on Conflicts and Breaks (#410)
- add test coverage for #410