Caching, move functionality to original plugins

- Expire cache depending on changes of the page, and the inserted pages
- Use different cachefiles for inserted pages with different headerindent levels
- Depend more on DokuWiki's export functionalities
- Move bibliography handling to Zotero plugin (not yet published)
- Move mathjax functionalities to Mathjax plugin (allready published) Fixed #19
- Remove some globals, store their properties in helper instance. See also #9
- Alternative wikipage insert syntax: 
  \insertwikipage{page}
  \insertwikipage[1]{ns:page}
  \insertwikipage[5]{ns:page|Title}
  For more robust caching handling.

- Rename class Package to LatexPackage Fixes #17
- More tests of inserting pages
- Some general code reformatting and refactoring