update FormAlchemy URL

http://formalchemy.org/ redirects to https://code.google.com/p/formalchemy/
https://code.google.com/p/formalchemy/ says:
    "Code has moved: FormAlchemy is now hosted on GitHub"
  and links to https://github.com/FormAlchemy
The FormAlchemy GitHub repo is https://github.com/FormAlchemy/formalchemy
Merge pull request #20 from cvrebert/patch-1

update FormAlchemy URL