Adds a get_record helper.

Adds a helper to return a single record object. Arguments are similar to
`get_records` and include:

* $recordType
* $params - An array of search parameters

This makes it a bit easier to query for and use a single record. Instead of
having to do this:

```
$record = get_records('Item', array(), 1);
$record = $record[0];
```

You can use this function:

```
$record = get_record('Item', array());
```

this kind of function was brought up in https://github.com/omeka/Omeka/issues/503#issuecomment-14867491.