Task results processing utils

The blueprint is aimed at implementing the rally.benchmark.processing.utils module and using it in different parts of Rally code. It includes:

1) The actual creation of the rally.benchmark.processing.utils module and implementing there several helper methods that process the task results data and return it in the aggregated form. This code will be mostly based upon the current implementation of the "task detailed" command. Thus, the implementation of the rally.cmd.task.TaskCommands.detailed() method is going to be changed as well to use the code from the utils.

2) The same utils code should be used for the "task plot2html" command, to enhance the output with the detailed information about benchmark runtimes in the form of tables (not only graphs).

3) The code used for populating the html graphs resembles the code for the "task detailed" command quite a bit. Thus, as the last step, it should be rewritten using the rally.benchmark.processing.utils module functions as well.