Keywords and keyword index

This pull request adds keyword support for the book.

# Changes

- 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
- A new "Keywords" tab, which only is visible when the "Include index" option is activated in the book AND tabbedview is installed and activated for the book.
- The "Keywords" tab provides a keyword selection (`select2`), showing the blocks which have the selected keyword; all links point to the content position in the book reader.

# TinyMCE plugin

![bildschirmfoto 2014-03-03 um 11 57 01](https://f.cloud.github.com/assets/7469/2319187/c806f936-a378-11e3-893b-2014a9756699.png)

# Keywords tab

![bildschirmfoto 2014-03-06 um 11 48 24](https://f.cloud.github.com/assets/7469/2344232/234f25ac-a51d-11e3-8603-b9420dbada14.png)

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Requires PR 4teamwork/ftw.pdfgenerator/pull/32