add annotate to environment

annotated user model with database schema

chapter 6.1.2 page 229
validate user name length and email format

6.2.4 page 248
enforce email uniqueness

index the database based on email address, for case-insensitive uniqueness

chapter 6.2 page 254
add password validations

chapter 6.3.4 page 265
Merge pull request #3 from bhouse/modeling_users

add annotate to environment