add test for inject_concurrency_blockers
this is where inject_concurrency_blockers will go
docs for PARALLEL_APPLY
have PARALLEL_APPLY default to True
create item._deps outside of inject_*
implement inject_concurrency_blockers
set PARALLEL_APPLY to False for package management items
make sure inject_concurrency_blockers doesn't insert deps when it shouldn't
rewrite inject_concurrency_blockers to work with other item dependencies
shorten existing tests
add test for flatten_dependencies
fixes for flatten_dependencies
make sure items is not a generator
make a copy of item dependencies

so item.DEPENDS_STATIC isn't modified
Fix double dependencies

With some bad luck, the user might have already defined a dependency
between these two items *manually*.
Merge pull request #41 from trehn/trehn/issue-38

PR for issue #38