Implement record deletion.

First checks if the record is already loaded in the store to prevent
additional requests to the server.

Then checks that the record isn’t dirty in case it is in the process of
being deleted.
Simplify deletion of records.

Use store.getById to simplify finding of records and use unloadRecord
instead of deleteRecord so dirty old records do not remain in the store.
Merge pull request #15 from nmec/feature/delete

Implement record deletion.