Use one Makefile

The main Makefile handles all compilation from the root directory.
Additionally main.c was renamed to sorting.c in order to make the
directory structure clear. The subdirectories will hold only algorithms
without execution logic.
Report results in JSON

Firstly, sorting algorithms are run only once and their results are
reported in microseconds instead of seconds.

Secondly, there's a Ruby script that executes algorithms specified on
the command line and produces a JSON file with the results. This file
will be used in HTML reports.