Merge branch 'release/0.3.0' into develop
Add `index_type` parameter to `MongoDict`. Fix #14

Also replace `value` with `v` in MongoDB documents to save space on large
dictionaries.
Merge branch 'feature/change-index' into develop
Fix problem in index test
Update log of changes to 0.3.1
Update version number
Fix a little problem in codec test
Add migration script and warning on CHANGELOG
Add migration script and CHANGELOG to MANIFEST.in
Merge branch 'release/0.3.1'