Right now we consider a file ".diceware" in the users home as
only valid place for a diceware config file.

Travis is not happy without `coverage`. It would be nice if we could
such requirements before commits. Maybe `tox` is the way to go then?
On the other hand it is nice to have different builds in travis.

Travis CI now comes with container infrastructure as shown in
http://docs.travis-ci.com/user/migrating-from-legacy/. Let's give it a
try.

Tell about dash by name.

when looking for valid wordlists.

Only return path if it exists and is valid.

Activates the -w option. Turns out we still have to fix
`get_wordlist_path` at it assumes that wordlists all end in
``.txt`` (which is not true).

The last hotfix to make sys.stdin seek()-able was far from being
proper. It checked only for sys.stdin (which is only one possible
stream delivering unseekable file positions) and didn't consider that
sys.stdin could be replaced (by somthing with proper seek
functionality).

We now test properly for working seek() functionality. If the stream
does not support it, we copy streamdata to a new temporary file. If
the file is not too large, we keep the file data in memory (up to 20
MB), so in most cases runtime trade-off should be moderate.

We have to try hard to create a string accepted by all supported
Python versions.

Add test case that provides a file descriptor with broken seek(). We
must expect "broken" seek() methods for instance with sys.stdin.

Use conftest module for fixtures we need in several test modules.

Input from sys.stdin is valid but not seekable. Cope with that.