allow changing the duration of the cache validity
move asserts in own functions, it's more readable that way
check whether a second try will yield a dunno result if the first was defer
clean up tested Twisted versions

add 14.0, drop versions not present in any decent distro
also add some comments
pep8 is a bitch
use trial -j2 when Twisted >= 12.3.0 is found

this cuts the trial tests from 40 to 15 sec.
increasing the number of threads further does not really help and
seems to produce false negatives.

Closes #15
disable parallel tests, this creates problems for generating IPs

Reopens #15
refactor greylisting tests

we do not need to check for a DUNNO on second try in all tests
better check once, but then actually for both results:
 - first retry too early, defer again
 - second retry OK

this should trigger fails for SQLite as described in #16
always use `datetime.datetime.now()`

different databases are not sure how current CURRENT_TIMESTAMP is
as we are always comparing with a loca `now()` anyways, let's use
it for the database too.

closes #16
let's not define a function for a one-liner, okay?
Merged changes from evgeni/bley commit 2e4676a