More info at unpinning

Ref #1303
Allow pinning dev-packages without a repo

Closes #941

Would be good to make specification of package name/version from the metadata
work directly (name conflict will fail with a message at the moment)
Allow installing absent package on "opam reinstall"

(after a confirmation)
Closes #1082
Better error message when .install fails
Fix 'metadata changed' on new pins
Change url for solver preferences to the opam one
Merge pull request #1340 from rdicosmo/update-url

Change url for solver preferences to the opam one
Merge pull request #1335 from OCamlPro/pin-without-repo

Pin packages without repo
Parsing of urls

Fixes one issue mentionned in #1285. This should handle git://, http:// etc. prefixes correctly,
as well as .git suffixes. There is still a heuristic that defaults to a version pin if the
target starts with a number and doesn't contain slashes.