Fix date verification

[Matthias Endler]

In #10, @kiwita88 discovered, that there is no date verification right now.
This means that dates like 32.08.2016 will not throw errors. We should fix that by creating proper dates from the parsed string, similar to this snippet:

from dateutil.parser import parse

a = "2012-10-09T19:00:55Z"

b = parse(a)

print(b.weekday())
# 1 (equal to a Tuesday)

[Matthias Endler]

This can be closed now thanks to @kiwita88's work in #25. 🎉