add the _DEBUG_LEVEL constant to control which debug messages are shown

[dealfonso]

functions p_debug, p_warning, p_error, p_debug_var were showing messages anytime. Using _DEBUG_LEVEL constant, it is possible to control which messages are shown depending on its value.

• 0: suppress any message
• 1: show error messages only
• 2: show error and warning messages
• 3 (or greater): show error, warning and debug messages. (*) default value

Usage: define _DEBUG_LEVEL constant before importing SAPP. e.g.:

define('_DEBUG_LEVEL', 2);
require_once('vendor/autoload.php');

[angeljqv]

@dealfonso normally DEBUG_LEVEL is 1 or 2 by default, why 3? Why always debug mode by default?

[dealfonso]

As it is a library, I find that it is more appropriate to deactivate the debug mode than activating it (at least, for the current state of sapp)

[angeljqv]

@dealfonso please release for packagist