add field created_at, updated_at to books table

src/database/models/Book.py

-from sqlalchemy import Column, Integer, String, Text, SmallInteger, Numeric
+from sqlalchemy import Column, Integer, String, Text, SmallInteger, Numeric, DateTime
 from sqlalchemy.ext.declarative import declarative_base
 from .Model import Model
+from datetime import datetime
 
 Base = declarative_base()
 
@@ -26,3 +27,6 @@ class Book(Base, Model):
     category = Column(String(32), index=True)
 
     currency_type = Column(String(4), index=True)
+    
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)

src/database/versions/3217c19ef3a6_create_books_table.py → src/database/versions/e19a8f6d44a2_create_books_table.py

 """create books table
 
-Revision ID: 3217c19ef3a6
+Revision ID: e19a8f6d44a2
 Revises: 
-Create Date: 2019-06-13 18:27:06.732796
+Create Date: 2019-06-17 13:21:39.815414
 
 """
 from alembic import op
@@ -10,7 +10,7 @@ import sqlalchemy as sa
 
 
 # revision identifiers, used by Alembic.
-revision = '3217c19ef3a6'
+revision = 'e19a8f6d44a2'
 down_revision = None
 branch_labels = None
 depends_on = None
@@ -34,6 +34,8 @@ def upgrade():
     sa.Column('count_reviews', sa.Integer(), nullable=True),
     sa.Column('category', sa.String(length=32), nullable=True),
     sa.Column('currency_type', sa.String(length=4), nullable=True),
+    sa.Column('created_at', sa.DateTime(), nullable=True),
+    sa.Column('updated_at', sa.DateTime(), nullable=True),
     sa.PrimaryKeyConstraint('id'),
     sa.UniqueConstraint('upc')
     )