Turn additional schema into a FormExtender

Don't actually change README yet or update tests.
WIP
Add a SchemaAdapter, use this to fix birthdate
Rewrite README to explain what the code is doing
set default birthdate, otherwise import handler breaks
capitalize birthdate field label
apply extenders only when addon is installed
do not include Accept Terms field into Add User form
proper newsletter field name
Rework README to hopefully be more clear
Add unit tests
Remove z3c.autoinclude

Otherwise it interferes with the plone.app.users tests, breaking them.
Remove form.widgets. prefix from userdata forms too

Fix up unittests to match change in p.a.users
We are no longer offering both formlib and z3c-form, so update imports accordingly
plone.formwidget.datetime uses Python datetime, selectboxes, update accordingly
PEP8-o-rama
Add CHANGES note, bump version, require p.a.u 2.0
Update form names to use default z3c.form prefixes
Correct import locations
Add .gitignore