minor readme updates
even more file closing cleanup
show pipestatus on failure
use textwrap.dedent to allow indenting strings written to files
add SlavedTree tests for inheriting categories from the master repo
disable pkgcore.test.package.test_base __slots__ to fix tests

Also re-enables __slots__ for virtuals since I'm not sure why they were
disabled in the first place. Doesn't matter much anyway since PROVIDES
type virtuals have been deprecated for a while now.