Fix bug in create schema that causes db upgrade failures

- When onupgradeneeded() is called in the original db.open() function, createSchema() will be called to update the db
  - Updating an existing 'table' will not work, however, as store.createIndex() fails on pre-existing indices
  - Adding a try-catch statement allows for checking if the index exists before calling store.createIndex()
    - This allows for updating by adding new indexes
    - This does not deal with modifying a pre-existing index
      - I am not actually sure if this case is even intended to be possible in indexedDB, will need to look into it more.
Merge pull request #86 from benjaminapetersen/bug-create-schema

Fix bug in create schema that causes db upgrade failures