Removed tag option for raw bibtex entry.
Added a note in the documentation on the usage of this option.

see #78

Avoids error:
```
Liquid Exception: Variable '{{ entry.abstract %}' was not properly
terminated with regexp: /\}\}/
```

see #59

see #44

In Jekyll's `_config.yml`, one should use the shown directive to load the plugin. Either `gem: ['jekyll/scholar']` or `gems: ['jekyll-scholar']` won't work.

fixes #37