Allow github support to be disabled in book.json

Fixes #238
Fix issue of disabling issues and contribute

Improve robustness of template display conditions
typo: "blockin" to "block in"
Merge pull request #240 from ajschumacher/patch-1

typo: "blockin" to "block in"
Exit process with -1 when error occurs
Print stack only when env DEBUG is set
Bump version to 0.4.3
Change advised ratio for cover
Add norm for small cover
Add more infos about gitbook.io
Extract title and description from readme before switching to multi-languages handling
Add support for multi-languages books in JSON format
Fix merge of output for multi-languages books
Improve parsing of readme (extraction of title and description as plain text and not markdown)
For multi-langs, run build one by one
Bump version to 0.4.4
Fix generate failing on README parsing

Failed if no title or description could be extracted
Bump version to 0.4.5
Improve grammar (thanks to @theoutlander )
Merge branch 'master' into livereload