Add DEBUG level logging

Motivation

Although SmartSprites tries to report errors as accurately as possible, sometimes it would be useful set SmartSprites to output those verbose messages that are not need for non-erroneous operation.

Implementation

Add a DEBUG logging level whose priority should be lower than INFO. In DEBUG logging level, SmartSprites could output:

• values of all parameters
• full stack traces of exceptions
• each directive parsed, with all values and location in the CSS file

The current default logging level (WARN) should remain unchanged.