Created new Configurations class.

This class contains a bunch of convenience methods that simplify the creation
of concrete configuration classes. It can be used when no complex
initialization is required and default settings are appropriate.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1617826 13f79535-47bb-0310-9956-ffa450edef68
Extended user's guide for file-based configurations.

Added a subsection about the Configurations class as an alternative for
full-blown configuration builders.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1617827 13f79535-47bb-0310-9956-ffa450edef68
Extended chapter about CombinedConfigurationBuilder.

Described how Configurations can be used to simplify the creation of a builder
for simple use cases.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1617828 13f79535-47bb-0310-9956-ffa450edef68
Missing svn properties.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1617829 13f79535-47bb-0310-9956-ffa450edef68