This is a tiny HTTP tool using Apache HttpComponents and based on Java 8, it
can be used to execute small http tasks like downloading the small web sites or
preparing testing data.

The HttpTask is a task of single request, while HttpJob is a work flow contains
multiple tasks. The basic structure of the Doggy is:
    Runner -> HttpJob -> (TaskName, PageParser)... -> HttpTask...
HttpJob will construct new HttpTask by the given pair of task name and page
parser. So the job will call the corresponding parsers to parse the page
based on the task name.