Dataset support, statistics fix, temporal subject fix

- Dataset support
  - Added new SobekCM type: Dataset
  - Updated the default templates to include that type
  - Added support for new MODS type of computer file, including subelements of type
    - Target audience  (existing)
    - Type of computer file ( new)
    - Government publication (existing)
  - Updated MARC record creation for new 008 option
- Fixed statistics issue where it was not calculating the difference between two arbitrary dates correctly
- Fixed issue reading temporal subjects without a string time period
Merge branch 'master' of github.com:MarkVSullivan/SobekCM-Web-Application