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
add comment
fix small typo
Merge pull request #950 from PyBossa/issue-empty-uploads-directory-installation

Add an empty uploads directory in PyBossa
Add tests for relative paths

Fixes #948
remove space from comment
fix error message
replace raises with assert_raises
Merge pull request #951 from PyBossa/issue-948-relative-paths-tests

Issue 948 relative paths tests
Merge branch 'master' into jobs-refactor

* master:
  replace raises with assert_raises
  fix error message
  remove space from comment
  Add tests for relative paths
  fix small typo
  add comment
  Add a test for local uploader standard directory
  Add an empty uploads directory in PyBossa