downloader: New module, future replacement for curl module.
switch from curl module to downloader module
Removed curl and curltargetlist modules.
debugging: Replace DPRINTF() by g_debug() from the GLib2 library.
Python: Add set_debug_log_handler function

This function provide a simple way to set a function that catch a Librepo
library debuging messages.

Example of usage:

    import librepo

    def debug_function(msg, userdata):
        userdata["counter"] += 1
        print msg

    user_data = { 'counter': 0 }
    librepo.set_debug_log_handler(debug_function, user_data)

Note: the user_data param is optional and could be omitted:

    import librepo

    def debug_function(msg, _):
        print msg

    librepo.set_debug_log_handler(debug_function)
README.md: Add note about available params for the C unittests