Add support for filtering farmers' market payments and products. Fixes #95.

Example requests:
http://ohanapi.herokuapp.com/api/search?payments=sfmnp
http://ohanapi.herokuapp.com/api/search?payments=SFMNP

http://ohanapi.herokuapp.com/api/search?products=cheese
http://ohanapi.herokuapp.com/api/search?products=Baked Goods
http://ohanapi.herokuapp.com/api/search?products=baked goods