Empty thumbnails creates warnings

[ryantology]

If you don't want to create thumbnails of your uploads (PDF, Word Docs etc..) and you pass an empty thumbsizes array (or leave the thumbsizes array out) you will get a warning notice.

This happens for both
// Verifies if the thumbsizes names is alphanumeric
and
// Create the folders for the uploads

There are two options that I see to solving this

1. Create a new option that turns thumbnails on or off
2. Added tests to see if the thumbnails array is empty