Add an empty uploads directory in PyBossa

Modified also .gitignore to not ignore the uploads folder itself
BUT ignore everything below the uploads folder!

Original idea on how to create an empty folder and ignore all parent file entries:
http://stackoverflow.com/a/932982
Add a test for local uploader standard directory

See also #948
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fix small typo
Merge pull request #950 from PyBossa/issue-empty-uploads-directory-installation

Add an empty uploads directory in PyBossa