Quick attempt at the first tier exercises

[thashepherd]

• Added genre field to Book. Now, running GET /books will return an additional genre field associated with each book:

{
	"title": "Cradle 2",
	"author": "Arthur D. Clarke",
	"publication_year": 1968,
	"rating": 5,
	"genre": "Science Fiction",
	"id": 0
}

• Added /byauthor and /byyear endpoints to allow filtering books by author (last name) and publication year. Note that /byauthor uses a simple string contains, so it's best to use either last or first names but not both.
• Added a (quick and dirty!) system for 'submitting' reviews for books, and deleting previously submitted reviews. Note that more refinement is necessary before this feature can go out, and that it would be more properly to submit it in a separate PR.
  POST /books/review/{book_id}:

{
	"title": "Shogun",
	"author": "James Clavell",
	"genre": "Historical Fiction",
	"publication_year": 1975,
	"reviews": [
		{
			"submitter": "Noah",
			"rating": 4
		},
		{
			"submitter": "Asta",
			"rating": 5
		},
		{
			"submitter": "Matt Alioto",
			"rating": 10
		}
	],
	"id": 0,
	"average_rating": 6.33
}

A DELETE /books/review/{book_id}/{submitter} endpoint has been included as well.