Fixed bug #26 that added a "JOIN (null)" statement when using NSDates in predicates

- 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.
Merge pull request #27 from veducm/master

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