Keywords and keyword index

This pull request adds keyword support for the book.
Includes:

- A tinymce Plugin for BookTextBlocks only, which allows to set keywords
- A keyword index `book_keywords`, indexing the keywords
- LaTeX changes which convert keywords to `\index{}` commands
- New field `include index` on books and a keyword index implementation in the default layout LaTeX layout

Includes PR #32 
Requires PR 4teamwork/ftw.pdfgenerator#51

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![bildschirmfoto 2014-03-03 um 11 57 01](https://f.cloud.github.com/assets/7469/2319187/c806f936-a378-11e3-893b-2014a9756699.png)