Fix various warnings

- Fix imports that are not used
- Fix exported variables from case or if statements
- The problems are important when warnings_as_errors is turned on
enable more warnings
prefer hackney to lhttpc

note: this is just a proof of concept, it may be better to patch erlcoud to
handle different clients instead so the patch can be accepted upstream.
Added support for updating endpoint attributes on SNS
Merge pull request #1 from benoitc/hackney

Use Hackney as lhttpc replacement