Check XHTML Content Docs extensions in EPUB 3

  * make sure the extension of XHTML Content Docs is '.xhtml', in `OPFChecker30`
  * remove duplicate `checkItem()` call in `OPFChecker`
  * Only report `HTM_014` for EPUB 2 (in `EpubHTML5StructureCheck`)
  * Add a new test for `HTM_014a`
  * Add a new test for `HTM_014`
  * Fix existing tests

Fixes #447