Add fill_out session for aborted answers documentation

Closes #6

LGTM given by: @marten

Squashed commit of the following:

commit 00ad9d0f1e8cc12081d7091d846c7d8608ff3389
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Tue Jul 1 10:51:44 2014 +0200

    rename to response_ids

commit 56ad19c9bc0265c9441d4da445c48f4e03efe332
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Tue Jul 1 10:48:44 2014 +0200

    move exception to separate header

commit 58b350eeca9a05f1f1d7cf38929fa8989db7e28b
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Tue Jul 1 10:43:06 2014 +0200

    tweak options

commit 2ed443b138ebeb698b6e7f8ee902af58b5a15f2c
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Tue Jul 1 10:40:41 2014 +0200

    add start for aborted answer docs

commit 78f61f94cb00390eb7f191590883287ad9777923
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Tue Jul 1 10:40:32 2014 +0200

    add id to docs

commit 2d98f0a00b4c642eec43b46ed68a4345325ce7fc
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Mon Jun 30 12:42:01 2014 +0200

    add aborted state

commit 1e844e5e2f9cd701a09fdafd60e0eb5006e2b3db
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Mon Jun 30 11:38:31 2014 +0200

    multiple questionnaire keys