Reorganize code, integrate worker

Split up the Flask frontend code and the backend processing (worker)
into separate files. Integrate the worker using multiprocessing.
Update readme
Improve css, minor fixes
Implement input validation

Using regex, validate the username input to be the standard username
characters (a-zA-Z0-9_) and the two domain fields to be either of form
(subdomain.domain.com) or (domain.com). (Fixes #1)