Merge remote-tracking branch 'OCamlPro/opam-features'

Conflicts:
	doc/dev-manual/dev-manual.pdf
	doc/dev-manual/dev-manual.tex
	src/client/opamClient.ml
	src/client/opamState.ml
	src/core/opamFile.ml
	src/core/opamFile.mli
	src/core/opamTypes.mli
	src/solver/opamSolver.ml
	src/tools/opam_mk_repo.ml