Typo fix 

Typo fix in threading_easy.py docstring: 'prescribed'
Add tf_idf to SFileFilter

This currently fails many tests! It is not yet meant to be merged.
Add tf-idf parameter to filtering in SFileFilter
Add tf-idf documentation
Remove unnecessary import
Remove stale code-comment
Clean tf_idf code and improve documentation/tests
Merge pull request #35 from rafacarrascosa/patch-1

Typo fix
Merge pull request #36 from ApproximateIdentity/tf_idf

Add tf-idf to SFileFilter