New FormBuilder class and support for joined retrievals.

Changes-->
1. Form builder functions removed from Model class. They are now in a
new FormBuilder class. The Model now contains an object of FormBuilder
class. All form builder functions now called through this object.
2. A new function in Model class for joined retrievals. Two tables can
now be joined with the retrieveRecordByJoin function. Check Model.php
for its documentation and their is an example in index.php in root
folder which demonstrates using this function.