Google OpenID support & some other small changes

I've added the following changes:
* Optional Google OpenID authentication support (enabled by setting the `GOOGLE_APPS_DOMAIN` value).
* Make the SSL verification on Graphite requests optional.
* Use `WTForms-Alchemy` instead of `model_form` as the later is deprecated and wasn't working properly with the version of SQLAlchemy I had.
* `requirements.txt` file.

Apologies that this PR includes so many things together... usually I try to separate to make merging easier.