Adding case-insensitive User.getByEmail method

fixes #1498

- emails are no longer converted to lowercase, local mailbox can validly
  be mixed case
- getByEmail uses JS to compare emails to ensure we can support unicode
- tests that users can be retrieved by their email address with case
  insensitivity
Merge pull request #1954 from ErisDS/issue-1498

Adding case-insensitive User.getByEmail method