Support System Property substitution in xml-config values

This allows system property substitution (like in Apache Tomcat) in
betterFORM's configuration XML. e.g., this allows the user to write
"${java.home}" instead of absolute paths when configuring betterFORMS
(needed this to reference Java's default keystore)
Merge branch 'development' of https://github.com/betterFORM/betterFORM into development
Merge pull request #24 from indeterminatus/development

Support System Property substitution in xml-config values