Previously, a logunit was considered a word only if it was all letters.  This is important for
tracking bigrams correctly.

Now, a logunit must have only at least one letter.  The dictionary check is still performed,
and punctuation, etc. still comes in as separate LogUnits.  But a word can contain a space,
which helps set up for logging words where spaces are inserted automatically, and other situations
in which text is committed with an additional space tacked onto the end.

Change-Id: Ia74094a99058890d20a9cdadf2d0989841a79a41