fix(wtf-call-tracing): ignore non-javascript scripts

Many libraries (e.g. mustache.js), frameworks (AngularJS) and languages (Dart) use script tag as a container element. In this case the script tag doesn't contain anything that WTF could instrument, so the element should be skipped altogether.
Add a warning when redifining events.

Creating an event will reuse an old event with the same name; warn the
user on the console when this happens.
Merge pull request #490 from mpd/master

Add a warning when redifining events.
Merge pull request #489 from IgorMinar/patch-2

fix(wtf-call-tracing): ignore non-javascript scripts