Add clear method, update documentation, remove pika.log, general cleanup.
Add clear method, update documentation, remove pika.log, general cleanup.

- Add Alexey Myasnikov's clear method which will make it into use in the Connection class.
- Replace pika.log with logging and getLogger
- Cleanup doctoring comments.

shinji-s/pika,vrtsystems/pika,reddec/pika,Zephor5/pika,benjamin9999/pika,jstnlef/pika,vitaly-krugl/pika,fkarb/pika-python3,zixiliuyue/pika,renshawbay/pika-python3,hugoxia/pika,skftn/pika,Tarsbot/pika,pika/pika,knowsis/pika