Added fix to issue #26 to avoid adding a "JOIN (null)" statement in predicates with NSDates

This implements a fix to the issue #26, taking into account the details explained in [this comment](https://github.com/project-imas/encrypted-core-data/issues/26#issuecomment-23789583).

- Improved the regex to assure the table name starts with a letter (as forced by Apple's Core Data).
- Check if relationship description exists before adding a new JOIN statement to avoid adding JOIN statements with null table names.