add file event handlers

No need to define the Mutex mutex_ as mutable there is no const method.

There's no need to define the Mutex mutex_ as mutable since class base_sink has no const method.

fix mongo_sink<std::mutex>::instance_ template

CMake: Support <PackageName>_ROOT

The policy_max in cmake_minimum_required() already does that.

Unhandled errors

inet_aton(), InetPton() return codes

Update to latest Travis CI Build Status

Found out the ~/github vdir portion of the URL is not needed.

https://travis-ci.com/gabime/spdlog (404's) → https://app.travis-ci.com/github/gabime/spdlog
https://travis-ci.com/gabime/spdlog.svg?branch=v1.x → https://app.travis-ci.com/gabime/spdlog.svg?branch=v1.x (Result from clicking Status Image url builder)

Add udp_sink

List Debian instructions in the README

Fixed qt_sinks ctor