Improved performance using tool function

This release:

• uses a much improved ("gpt-v2") prompt. Among other things, the prompt offers a tool function that allows the LLM dynamically request additional context about the source code;
• includes full JSON dump of exchanges in log files;
• provides a tool module, coverup.logreader, that reorganizes events in log files, so that full sequences for each code segment can be viewed in order;
• renames --source-dir to --package-dir, to better indicate what it does;
• improves interactions with pytest-cleanslate;
• makes the Prompter interface more flexible, allowing for more customization;