Pay more attention to order of elements (e.g. entries) on output

Many feed readers will present the entries in the order they are
given.  For the user of this library to be able to influence that
order, the items must be outputted in the same order they are listed
in the feed.entries field.
Merge pull request #42 from Chris00/master

Pay more attention to order of elements (e.g. entries) on output
Add safe_string to _tags

This is in preparation for OCaml 4.02.0
See https://github.com/ocaml/ocaml/pull/86
Bump the version in _oasis

Also add an author — because it was added by hand to the OPAM package
https://github.com/ocaml/opam-repository/pull/2456/files
Add constructors for Atom.
Remove the unit argument in mk_source and reorder arguments.
Remove the `mk_` prefix.
Fix travis script
Fix version of uri
Downgrade uri
Update README
Merge pull request #47 from Cumulus/fix-travis

Fix travis script
Merge pull request #46 from Drup/constructors

Add constructors for Atom.
Merge master into gh-pages
Update documentation 128