Metadata extractor and Maven updates (saxon94 branch)

* Update metadata extractor to use version 2.6.2 of the library

* Updated Maven build configuration
  * Change the `<artifactId>` for the maven release to `xmlcalabash-saxon94` to eliminate conflicts for parallel releases targeting both Saxon 9.4 and Saxon 9.5
  * Self-contained `pom.xml` (specifies version and Saxon reference version directly)
  * 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. This is disabled by default; when the `sonatype-oss-release` profile is explicitly enabled you need to make sure and specify the path to Java 6 per the instructions given at [Building ANTLR 4 with Maven - Bootstrap Classpath](http://www.antlr.org/wiki/display/ANTLR4/Building+ANTLR+4+with+Maven)
  * Use recommended parent POM for Sonatype OSS releases
  * Update optional dependencies, and exclude optional functionality in cases where the dependencies are not publicly available