Calculate row and column using `vertical-motion'

* company--replacement-string: Account for horizontal scrolling.
* Fixes #22.

Thanks to Eli Zaretskii for suggestions:
http://lists.gnu.org/archive/html/emacs-devel/2013-05/msg00145.html
Check in NEWS and tests for the previous commit
company--column: Account for character composition
Respect line-prefix at bol (#24)
Fix #24, part second

Regression from a696ca3a.