HTTP client changes, bug and documentation fixes

• Remove default dependency on ibrowse by changing the default HTTP client to inets httpc
• Configuration support for hackney HTTP client next to ibrowse
• Support for HTTP request callback, allowing the use of other HTTP clients
• Add description of using OTTER with Jaeger trace collector
• Use otter.hrl from otter_lib/include instead of otter_lib/src to make mix happy
• Merge documentation and bugfixes