Optionally convert username to lowercase before find

Given that:
 * I am using email addresses for usernames
 * email addresses are generally case-insensitive (or converted to upper/lower case)
 * I use the lowercase option on my schema to convert usernames
 * I want my users to be able to neurotically mix upper and lowercase letters in their email addresses when logging in

I have made these changes:
 * created `options.usernameLowerCase` which defaults to `false`
 * if `options.usernameLowerCase` is `true`, convert username to lowercase in `schema.statics.findByUsername`.