Fixed issue with some helpers not being loaded.

The issue comes from an initializer that was hijacking the loading
process. It seems I overlooked a place in rails that was already doing
what I wanted and offered a cleaner approach. So I changed my code to
reflect that algorithm and it fixed all the loading issues.
Re-added S3 support and automatic upload

Images used to have their own section but this was replaced with a more
inline alternative. When creating an <img> tag in the textarea,
Ecrire will replace it if it has no src="" attribute with a form. That
form allow you to either click on it and select a file via the popup or
drag & drop an image. Once it's uploaded, Ecrire will replace the form
and them empty <img> tag with the image

This fix #52