Feature - System models and design

[ParthBarot-BoTreeConsulting]

As per issue #2 and #5, We need to add following models.

1. Company - has_many users, has_many clients, has_many invoices
2. User - has_many invoices & clients through company
3. Client - has_many contact_details, has_one address, has_many invoices
4. Address
5. ContactDetail
6. Invoice - has_many line_items
7. LineItem - has_many taxes
8. Tax
9. Currency
10. Country

Please add columns as needed, as shown in related issues.

Others

1. Devise gem for user authentication.
2. Cancan for permissions to diff. aspects of the company data, for diff. users - Later