Added failing unit test for digram chain

This will requrie a major rework of makeChain to pass.  The problem is
dynamically indexing arbitrary deep into the chain.  For example:

root [chain] [former[level]] [chain] [former[level-1]]...

Until _level_ - _n_ is 0. For _level_ in range(_grams_).  Where
_former_ is a _grams_ - 1 long array of former tokens.

Wonder wether python has a facility for this (macros?)  (Generating the
bytecode is out of the question.)  I should read up on zippers. But I'm
open for any suggestion.

note: Also we should put a terminating element (None) only at the
lowest level. This would mean, that if the input is shorter than
_grams_ - 1 we never terminate. Should address this.