Change default value of dmax to 0

This affects jmxetric.xml and jmxetric_test.xml
The original default value was "", resulting in an exception when
trying to parse it into an Integer

This is related to #22 where there is a NPE when
XMLConfigurationService tries to call Integer.parseInt.
Add parseDMax to safely parse the value of dmax

The value of dmax read from the configuration file may be
an empty string, which causes NumberFormatException.
parseDMax catches the exception and returns the default value
of dmax, 0.

Fixes issue #22.
add default constants for XMLConfigurationService
fix up comments
throw more specific Exceptions