Make sure that config file values are really considered by
handle_options().

We pin py.test to version < 2.8.0 to make it work with the
`--doctest-modules` option enabled. We still wait for this bug
in py.test being fixed (announced in issue tracker for 2.8.3
release of py.test).

Give hints, how to customize configuration.

As we have new features in the upcoming release, we have to bump the
minor release number, not only the bugfix number.

We do support configuration files from now on.

As we introduced an py.test autouse fixture, we cannot currently also
run doctest-modules due to a bug in py.test (2.8.1, 2.8.2):

   https://github.com/pytest-dev/pytest/issues/1057

Looks like a fix is planned for 2.8.3 release. Until then we have to
disable doctests in modules.

With the new config module in place we must make sure, that any user
maintained .diceware.ini files do not interfere with tests.

The fixture sets $HOME for all tests automatically.

This test name was a duplicate, in effect overwriting the other test.

We also have to check the "True" case to make the test relevant.

We have to check for boolean before int, because int is also
a valid boolean.

Older python versions (< 3.x?) do not support item
assignments with config parser instances. We have
to workaround therefore.

By default we should read config files in user
homes.

We can build a sensible list of config file locations on-the-fly.

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.