Update persistent_identifiers.md
Merge pull request #22 from fscheiner/master

Fixes #21.
Updated ChangeLog.
Merge pull request #24 from giotti/patch-1

Update persistent_identifiers.md
Corrected typo.
Merge branch 'master' of fscheiner-at-github.com:fr4nk5ch31n3r/gtransfer
Incremented version number.