Refactor reporters

Move reporter code out of tracer.js
Add default reporters
	- _default: throws the exception
	- longStackTrace: builds and displays stack trace with async calls

Users can override the list of reporters using

    zone.reporters = [Zone.reporters.longStackTrace];
Use global Zone object to store reporters
Use single filter

Stack was filtered in multiple places. Use only one.
Remove lines which only contain 'Error'
Expose reporter longStackTrace
Merge branch 'refactor-reporters'
version bumped