Make sure that strings/vlens read by HDF5 are getting garbage collected

[simonster]

The fact that the HDF5 tests weren't causing segfaults on Windows before we changed the libhdf5 binary makes me worry that strings and vlens read through the HDF5 interface might not be getting garbage collected. I think JLD should be fine.

[timholy]

Yeah, I too was wondering why we hadn't had a problem before. If you don't beat me to it, I'll gladly take a look once I finally shed my own Sisyphean task, JuliaImages/Images.jl#135.