Updated license-header in sources.

- Added a new file called license-header.txt to contain the header that
  will be used for all source files. The file contains a configurable
  ${year}-variable that can be set from within the
  maven-license-plugin's configuration in the pom.xml file
- Added the maven-license-plugin to the main pom.xml file. Configured
  the plugin to read the license-header from the license-header.txt file
  in the root directory. Added includes for all of the sub-module source
  directories. Also, configured the styling differently for java source
  files versus html files.
- Updated all of the licenses by running the 'mvn license:format'
  command.