Version 0.6.0

Bugfix and feature release (hence the version bump).

The main change is to the implementation of how attributes are listed and read. This was done due to existing race issues when attributes are modified while being read (github issue #12), but basically all various internal paths that dealt with retrieving an attribute value or listing attributes were unified in a single helper function that does handle such concurrent modifications. As a side effect, the size of the buffers used for such reads have changed, which (depending on attribute value) might change the trade-off between number of syscalls done and memory usage.

As feature release, OSX support was contributed by Adam Knight, thanks a lot! I don't have access to OSX so the testing for it is done via Travis builds; please report any issues.