Add reference links support

This commit resolves #184
fix links error
Improve code syntax for links assignment
Use ebook-convert for pdf generation
Generate valid first level table of content for ebook
Improve ebook generation (toc, ...)
Add toc at the end of pdf
Add margin on pdf output
Adding a new plugin.

I Just published a first version of the Piwik Open Analytics plugin, on both GIT and NPM.
Merge pull request #274 from emmanuel-keller/patch-1

Adding a new plugin (Piwik Open Analytics plugin)
Force paper format to A4 fro pdf
Set default pdf font size to 12
Bump version to 0.4.8
Add error message when generator is invalid

Fixes #276, #277
Fix error during generation
Improve documentation about book.json
Fix error when plugin don't extend book
Fix #282: stop generation of langs index for page format
Langs index page now use direct link to html file
Add options pdf.toc and pdf.fontSize