Bindoc ext

Test case for #32 (should fail for now)

some notes:

1. I added bindoc_dirs to test. not sure now that the fact that M::B should work without bindoc_dirs tested elsewhere.
2. not sure what should happen if you have `bin/foo` and `docs/foo` at same time (and when we fix #32, what should happen if you have `docs/foo` and `docs/foo.pod`), so not tested this.
3. test code did not work properly with cases `docs/foo.` (period in the end), so I did not include tests for it.