Fixes #382: Issues with spack fetch.
Fixes #382: Issues with spack fetch.

- urljoin() was compliant with RFC 1808 but not with my understanding
  of how paths should be joined.
- updated path joining logic to comply.

LLNL/spack,EmreAtes/spack,EmreAtes/spack,EmreAtes/spack,krafczyk/spack,mfherbst/spack,TheTimmy/spack,mfherbst/spack,lgarren/spack,skosukhin/spack,matthiasdiener/spack,matthiasdiener/spack,tmerrick1/spack,LLNL/spack,matthiasdiener/spack,TheTimmy/spack,matthiasdiener/spack,EmreAtes/spack,krafczyk/spack,matthiasdiener/spack,tmerrick1/spack,mfherbst/spack,tmerrick1/spack,LLNL/spack,lgarren/spack,mfherbst/spack,TheTimmy/spack,LLNL/spack,lgarren/spack,krafczyk/spack,EmreAtes/spack,iulian787/spack,skosukhin/spack,skosukhin/spack,krafczyk/spack,lgarren/spack,TheTimmy/spack,mfherbst/spack,TheTimmy/spack,krafczyk/spack,lgarren/spack,tmerrick1/spack,iulian787/spack,LLNL/spack,iulian787/spack,skosukhin/spack,iulian787/spack,tmerrick1/spack,iulian787/spack,skosukhin/spack