Disable the REST Upload API by default

Hii Bernardo,

Apparently there's no way to make a pull request for wiki pages in GitHub. Coud you manually copy the content from my fork : https://github.com/nhuray/thumbor/wiki/How-upload-images-in-thumbor.

Moreover I submit this pull request cause currently the REST Upload API is enabled, so it will be better if the end-point is disabled like the /upload end-point.  

Regards,