Closes #17; text is not merged across vertical_rulings

Also, when non-vertically-overlapping text in the same column is merged, a space is added.

FWIW, lib/tabula/table_extractor.rb is a real mess and needs a refactor. There's some duplicated code in the methods I've added (e.g. `make_table_with_vertical_rulings` and `merge_words_in_a_vertical_rulings_aware_manner`). These methods and their non-vertical-rulings-aware counterparts could definitely be combined into a single method that behaves differently based on the contents of `options`.

Take a look :)