Some methods refactored to support more formats using the protocol StringLiteralConvertible

### What
Some methods related to the finder using the strong type **String**. Instead of that I've refactored those methods to use **StringLiteralConvertible**. That way the user can pass more types to initialize the predicate, or the SortDescriptor.

Resolves #85 

### Tests?
- Nothing to be tested. The existing tests are valid