Merge pull request #505 from euanh/downloader

downloader: Update access time for existing archives
opam: Add initial spec file

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #506 from euanh/opam

opam: Add initial spec file
opam: Add documentation and remove opam-installer

Signed-off-by: Euan Harris <euan.harris@citrix.com>