Add a common place where components can write messages to. Any
userinterface can decide, what messages should be visible
(depending on their severity).

With one item in sequence we do not have to roll dice. This holds for
all sorts of dice.

The moved comment was at the wrong place.

I guess it is obvious, how the first if-clause works.

We can simply do the (cheap) modulo operation after each die role.

Use multiples of 4 as indent, shorten overlong lines and fix
other minor things to keep pep8-conformity.

Move the calculations out of the `pre_check()` function.
Also do smarter checks:
  - If len(sequence) == 1, we don't need any rolls. In this case, just
    return sequence[0]
  - If len(sequence) is a factor of dice_sides, we only need one roll
    along with the modulo operator.
  - For everything else, just require one extra roll, bringing it to a
    total of 2 rolls. This should remove a little bit of the edge bias
    introduced by the modulo
Run the `pre_check()` function after the calculations have been done.

For the unit tests:
  - remove the function `test_choice_len_too_short()`;
    we no longer have to raise an exception when the sequence length is
    too short.
  - Write new tests for the cases where `dice_sides = 6` and `len(sequence)`:
     - 1
     - 2,3
     - 4,5

Add an if to the pre_check that catches cases where there are less items
to choose from than sides on the die. If this is the case, we can still
pick items, but we have to use a modulo.
Pick the `num_rolls` so that the result generated after rolling is larger
 than `100*len(sequence)`.
The `pre_check()` function now has to return the modified `num_rolls` as
well as a boolean to indicate whether modulo needs to be applied on the
result.

Use `.asc` filename extension for signed wordlists.

also updated README

Fixes #16

Normally, it would be more concise to use the more declarative name.
"en_8k" tells, how many terms we find in this wordlist. But as the
wordlist names are used in the CLI and users will normally simply
look for a wordlist in their language, it can help to keep the name
of the "default" list for each language short.

We pick the 8K lists for "default" (and not the "original" 6^5
lists), because we assume that normally people will use diceware
without dice. Also most other sources of randomness will most
probably work on a 2^x basis rather than in 6^x (and therefore
have better use for 8k lists over other ones).

We do support configuration files from now on.

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.

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.

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).