Added a couchDBValue(...) repo method for converting view index values

When using a value that is converted when writing to a CouchDB document, mainly `DateTime` objects, as the key of a view the value used as index into that view must also be converted. (Both in user defined views and calls to `findBy()`). The method `DocumentRepository::couchDBValue(...)` has been added to conveniently handle this conversion within a repository by accepting the field name and value to be used as index.