Indexed db support

This adds indexed DB support in the form of two new export methods which will allow you to export partial data sets and data sets by type.

Example (export all of the entities from this manager and upset them to indexed db):
(this is an angular example and uses angular-indexedDB
```
  #coffee
  types = _(entityManager.metadataStore.getEntityTypes()).reject (t) -> t.isComplexType
  for entityType in types
    store = $indexedDB.objectStore  _(entityType.shortName).pluralize()
    exportedEntities = entityManager.exportEntitiesByType entityType
    store.upsert exportedEntities
```