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
slight formatting changes
Fix regression introduced by 78fb5c7

The commit introduced default values for configuration parameters that
can be specified on the command line, but it set those default values
too early.
This led to it overwriting those values when they are specified in the
XML file.
This change changes it so that the absence of a command line parameter
leads to it being set as null, rather than the default value.
The default value is set during the last stage of the Ganglia
configuration, which is after the XML file has been read.
Merge pull request #26 from ngzhian/config-file-regression-fix

Fix regression introduced by 78fb5c7
Updates to README, version and installation

Bump version to 1.0.7.
Add in more details to Demo/Quickstart section.
Merge branch 'readme-fixes' of github.com:ngzhian/jmxetric into readme-fixes
Remove version number for jmxetric and gmetric.

The rationale for this is discussed in the following commit on GitHub:
https://github.com/ngzhian/jmxetric/commit/e0a05aa5a39815dce58a7401095f6bead3f41152