Check datadog if class exists

[erickhun]

This PR:

• check if \DDTrace\GlobalTracer (is installed by datadog apm extension), exists
• We also catch the exception if \DDTrace\GlobalTracer::get()->getActiveSpan() throw an error
• should solve Potential unexpected nulls #3 dd-trace dependency #4 Check if DDTrace/GlobalTracer tracer exists #10

@colinscape up for a review? :)