Update/improve routes documentation

[rrickgauer]

• Add all the available routes to the README
• Add all the available routes to the project's __init__.py file

[rrickgauer]

Resource	URI
User	/users/:user_id
Products	/users/:user_id/products/:product_id
Product Availability	/users/:user_id/products/:product_id/availability/:product_availability_id
Product Images	/users/:user_id/products/:product_id/images/:product_image_id
Listings	/listings/:product_id
Product Categories	/product-categories
Login	/login
Locations	/locations/:location_id
Search Locations	/search/locations
Search Products	/search/products