Feature/gistify

Please review this code before merging. Also please **don't** merge with fast-forward, so the branch is not destroyed.

# New dependencies
- Composer (require): facebook/php-sdk-v4

# New requirements
- A Facebook App is required. For open groups the App Secret and App ID are sufficient (see parameters.yml). For closed groups an User Access Token may be required, although this hasn't been tested yet.

# Change log
- Added Facebook Graph connectivity
- Added module for handling new posts
- New posts are screened for code content (>= 3 subsequent lines)
- Posts with inline code get a comment reply + gist link to the gistificated code

# Future improvements & justification
Currently the NewPost module has only one feature: gistify. This feature is directly weaved in the module. In the future many more features may be required, so there will be a need for separate module handlers as described in the architecture @ issue #1 .