Convert to Gradle Build

Previously the build was using Maven. The build now uses Gradle to
make it easier to perform releases with the Spring ecosystem. A few
things to note:

  - ${version} is now used in the documentation to keep the doc up to
    date
  - The reference source was broken up into multiple xml files to allow
    the Gradle docbook plugin to work (the file was too large otherwise)
  - The generated doc was removed as it will be part of the CI build and
    automatically deployed
  - The reference was updated to describe how to build with Gradle
  - The dependency slf4j-api is used explicitly and so an explicit
    dependency was added