Add test case and fix for #280

Reviewed-by: javajolt

This provokes the failure but I am having a hard time
capturing the failure. When run from the shell the
message {'gen_event_EXIT',Handler,Reason} is sent
but within the context of eunit it doesn't seem to be
going to the right mailbox.

Integrate patches into lager 3.0 release

Based on code review comments put this function into util.
Added a couple simple unit tests for sanity checking purposes.

Removed comment

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.

Add an unsafe_formatting code path

Reviewed-by: JeetKunDoug

Goldrush 0.1.7 provides for additional comparison operators in traces.

Fix stop_trace_int so it can remove an event handler

Previously bad configurations would be
silently ignored. Now they will throw
an error tuple.

Previously, would fail due to lager:get_loglevel/2 not being exported.

Originally I tried to bodge in a handcoded parse transform and it
did not go very well.  This time, we simplify the case statement
in lager.erl and use this simplified version in the parse transform
after getting the AST from the Erlang compiler.