Make "def width" a private API; expose isWidthKnown instead (#257)

* Make "def width" a private API; expose isWidthKnown instead

Resolves #256.

Since width was used to determine whether getWidth would succeed, I added

    def isWidthKnown: Boolean

but another option would be to expose something like

    def widthOption: Option[Int]

...thoughts?

* Document getWidth/isWidthKnown

* Add widthOption for more idiomatic Scala manipulation of widths