Add dummy words to stay comparable when needed

Documents might contain slightly out of sync token structures, when
artificial tokens were inserted in one but not the other.

To keep the program running, we enter something like an empty word,
which is effective, but can also be problematic in the final reports:
When a word is present only in the reviewable file, we cannot keep
track of it in the diff report.
Capture inexistant attributes in Word

... and put something that indicates emptiness ('-') there.
Add specs for elliptic stuff
Add specs for files with missing annotation
Cover up empty postags
Refactor Word#[], which also enables postag fixes
Add specs for missing postags
Mark inexistant filled-in tokens
Merge pull request #34 from latin-language-toolkit/recoveries

Recover from various issues