Standardize a login/accounts package

[pjz]

Model functionality on meteor's: basic but with an API to allow plugins.

'pip install aspen-accounts' should get it, with either default simplates provided or being so simple (1-2 LOC) that they can be done entirely in documentation.

[pjz]

At least do the integration with authomatic

[pjz]

Now to be handled by the framework (flask, django, whatever) instead of Aspen directly