Request HTTP compression and obey Content-Disposition if possible

Also modifies OpamFilename.download semantics to allow the ~dst
to be a directory. Fixes OCamlPro/opam#757.
Merge pull request #816 from dsheets/http_conneg

Request HTTP compression and obey Content-Disposition if possible
Colorising opam switch list
fix: opam pin xxx.git#master
Added an option to update repos only (no dev packages)
Display backtrace when exiting with error code and debug is set
Merge pull request #817 from AltGr/meeting-requests

Meeting requests
Fix git remote tag packages
oops, my mistake
Merge pull request #820 from AltGr/fix-git-tags

Fix git remote tag packages
Revert "Request HTTP compression and obey Content-Disposition if possible"

This reverts commit 51e7418ec53724d6859ae339d3a4d8109f3443d7.
Breaks http downloads on some of our setups, reverting until we investigate.