fixes #27
don't try and calculate tf for empty fields

was causing issues when adding a document with an empty field, 0 / 0 would give a NaN which would end up as the tf, and then as the documents score against a query
Merge pull request #31 from olivernn/pr/28

Fix bug when indexing a document with empty fields
build version 0.3.2
Merge branch 'master' into gh-pages