Add single() and singleOrDefault() for query builder

[theminer3746]

Currently, Laravel has first() which returns the first result, firstOrFail() which returns the first result or throw an exception if no result was found, and firstOrDefault() which returns the first result or a default if no result was found.

I propose implementing single() which returns the first result if there is exactly one result and throw an exception if there are many or none. singleOrDefault() works in similiar way but returns a default value if no result was found and throw an exception of multiple results were found.

The method name and general idea was inspired by LINQ.

[koenhoeijmakers]

This already exists but its named sole()