add stringext as cohttp dependency explictly

currently cohttp relies on uri to pull it in but if uri ever drops it
then cohttp will break because it depends on it explictly.
+dnscurve.0.2.0
+sodium.0.2.1
Merge pull request #2467 from rgrinberg/cohttp_stringext

add stringext as cohttp dependency explictly
Merge pull request #2469 from dsheets/dnscurve

+dnscurve.0.2.0
Merge pull request #2466 from rgrinberg/master

opium 0.10.0
Merge pull request #2470 from dsheets/sodium

+sodium.0.2.1