Started implementing factor support
Finished as.factor method
Added factor part into extraction methods
Added factor part into replacement methods
Allowed adding new columns using $<- and [[<-
Made as.db.data.frame accept factor
Finished as.db.data.frame

Also added .where property into definition of db.Rquery class

Improved extraction and replacement methods with the new .where
property.
Fixed a bug in create replacements
Re-fixed the previous bug

The previous bug is not actually a bug. It was correct.

The original column is still useful (for grouping column for example)
    and so it should be kept.
Corrected another bug in replacement methods

.expr should use x's .expr as the original values
Working on formula analyzer