Jaccard distances calculation added
Speed up & doctsrings

Speed up kudos go to pre-created insert list s_distances &
load_bulk=False
Add tags filtering

Only tags set by 2 or more users are being considered since now.
Add indexes to ShredsDistances
Code clean up
Code generalization & .gitignore
Add CLI command to calculate Jaccard distances
Add view with shred pairs ordered by Jaccard distance
Small fixes & improvements

Extract shreds distance view to blueprint, remove outdated docstring &
prevent wiping all distances in future.
Merge pull request #59 from z4y4ts/master

Jaccard distances calculation & review view.