Updated Maven build configuration

* Use separate artifactId values for the Saxon 9.4 and Saxon 9.5 releases
* Specify source file encoding as UTF-8
* Updated issueManagement and scm sections
* Support verifying release builds against Java 6, even when building using the JDK 7 compiler
* Use recommended parent POM for Sonatype OSS releases
* Update optional dependencies, and exclude optional functionality in cases where the dependencies are not publicly available