[pyramid] trace the app when the tween list is explicitly specified (#349)

* Fix explicit tween definition bug
* Refactor pyramid tests
* More tests, better tests
* Do not trace render when request is not traced
* Enforce logging when DATADOG_TRACE_DEBUG is set
* Fix naming for consistency
* More doc on how to initialize tweens
* Extracted span field name as constant
* Put constants in variables
* Renamed tween_list to tweens since it's not a python list
* Comment to clarify relative tweens positioning
* Terser code for handling special cases in patch.py