This somewhat breaks backwards compatibility, but not for the
previously documented arguments: floats are now interpreted
differently, but ints and datetime.datetime objects are interpreted
the same as before. But the documentation clearly stated that only
ints and datetime.datetime objects were allowed.

This makes seek_realtime match seek_monotonic and other functions
which take time and follows the principle of least surprise.

Fixes #21.

Let's pass any flags through to the journal functions without checking
validity. Those functions do their own checking, so there's no need to
second-guess.

The semantics for _Reader(flags=0) are a bit changed:
before, this would be transformed into sd_journal_open(SD_J_LOCAL_ONLY).
Now, this results in sd_journal_open(0). Previous behaviour should be
achieved by not specifying the flags at all.
This change is necessary, because previously it was not possible to
pass flags=0 to sd_journal_open(0), i.e. it was not possible to "merge"
journals through the Python interface.

Similarly, Reader(flags=0) now means to open all journals, and
Reader(flags=None) is the same as Reader(flags=LOCAL_ONLY).

The socket module seems to be missing this in python2.7.