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
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.
Don't put logs in ~/.opam when used as a library

This initialises the root dir in /tmp until opam's cl options are parsed.

Closes  #1355
(also includes small fix to #1352)
Fix opam-check for initialising opam-root
Merge pull request #1344 from OCamlPro/parse-urls

Parsing of urls
Merge pull request #1356 from OCamlPro/opam-installer-home

Don't put logs in ~/.opam when used as a library
Add some convenience printers in OpamCudf

Now that the opam-name and opam-version are in the CUDF file, it's much easier to debug. Sweet!
Some tweaks to the heuristic to work on smaller universes.

This change makes opam-rt work again with the internal heuristics.
Merge pull request #1358 from samoht/master

Fix the internal solver to work decently again.
Fix handling of ftp urls
Merge pull request #1360 from AltGr/fix-ftp

Fix handling of ftp urls
Remove the complex and unused `opam config {asm,byte}{comp,link}`
Remove the complex and unused `opam config -I` command
Simplify the libraries and syntax fields

Show them on `opam show`
Simplify the structure of .config file

No more subsections, only a list of variables.