added loan and loan_request model

added validations for all models

Added a loan_request_model that is a join table
between category and loan_request. This represents
a many-to-many relationship.

Also added validations in the loan_request_model

updated the database screenshot
Merge branch '14_12_11_create_models' of https://github.com/bayendor/the_pivot into 14_12_11_create_models