comment all test codes in crowdscreen.py
remove solver from __init__.py
delete PSolver.py
modify some printing function
modify .gitignore to ignore files with names start with "tmp"
Modify the assigner interface, add StrategyAssigner ...

1. modify evaluate(), and add getConfusionMatrix() and getBinaryTaskMetrics
2. add interface update() and abandon() for assigner, the interface get() is renamed to assign()
3. add some customed exception for assigner
4. add assignerDeducer for the assigners which deduce true answer by themself
5. add PWorker2, which solve the task based on false positive/negetive rate.
add plotPR.py