Added date range and date validation for signup

1. Added date range for month and year.
2. Added Python date validation for date.

I think the underlying problem here is [this bit of code](https://github.com/praekelt/ghr-mobi/blame/develop/src/app/authentication/models.py#L63-L74) you've added.

Why are day, month and year defined as CharFields and not as integers? A South data- and schema migration can fix this and this means you don't need to worry about maintaining these kinds of quirks through the codebase.