Improved debug output facilities

Three logging functions were added:
 * logInfo
 * logWarning
 * logError

preparing ostream with colorful prefixes, ready to use.

The nDebug macro was improved to support both configurable color
and several verbosity levels.

To specify verbosity, use nDebugL(lvl). For convenience and compatibility,
there are the predefined mappings
 * nDebug -> nDebugL(1)
 * nDebugVerbose -> nDebugL(2)

The integer value of DEBUG sets the level to output. DEBUG_COLOR controls
whether colors are used.