Merge pull request #834 from samoht/master

More fixes
Writing switch backups to allow for rollback after eg. failed upgrades
Merge pull request #837 from AltGr/rollbacks

Writing switch backups to allow for rollback after eg. failed upgrades
Fixed git diff
Closes #836. Thanks for reporting !
Merge pull request #838 from AltGr/git-pin

Fixed git diff
Fixed warning at update about missing opam files on installed pinned packages
Don't repeat the [ERROR] tag at end of command
Merge pull request #839 from AltGr/fix-some-warns

Fix some warns
Always display installed-version in `opam info`

Fix #752
Merge branch 'master' of github.com:OCamlPro/opam
Fix #806

Still need to double-check
Add 'opam-admin stats'
Fix #842
Tweaks to 'opam-admin stats'
Update CHANGES
More lenient parser

Useful for forward compatibility
Add a 'bug-reports' field. Useful for OCamlot triaging.
Doc, License, Homepage and Maintainer fields can now take a list of strings