Implement a simple rspec test

Will allow testing more complex functionality
Handle flags even if no other headers

 - add test for sampling with/without header
Add plugins for annotation, whitelisting, and filtering

 - annotate_plugin: add annotations based on Rack env +
   response status, headers, body
 - whitelist_plugin: force sampling if condition matched
 - filter_plugin: skip tracing if condition matched
Update to v0.3.0 with plugin support