Emphasize application variable

* A side effect is you can set per processes functions via lager:md/1

* Updated README with instructions on how to use
 * Basic set of tests for working and non working examples

It wasn't clear whether "it" referred to async_threshold or
async_threshold_window.

Addresses #287

During code review we decided that using Sink ++ '_lager_event'
would provide better isolation for gen_event names than
Sink ++ '_event' as we had been using up to this point.

For example a sink called 'audit' is now 'audit_lager_event'
than 'audit_event' which might clash with a user's predefined
own module or process name.