Added CONTRIBUTING.md.
updated javadoc on visibility
update Props javadocs, clean up api calls
Added ability to customize trace data captured for debugging purposes.
Only put API method in a trace when api boundary is found

This commit makes TraceUtil only include the user code's location
in a trace by default. It only becomes "apiMethod() at code_location"
when a registered API boundary is found. The result is that "naked"
Cascading's behavior doesn't change at all but frameworks that
register api boundaries can get additional information.