Turn banner truncates road names

[1ec5]

Long road names or road names that include long words easily overflow the turn banner, which can only accommodate two lines of text. We should set the UILabel to max out at two lines and start shrinking to fit for anything that doesn’t fit on two lines. We should also progressively abbreviate long labels.

The current road name label added in #155 pulls the road name from Mapbox Streets vector tiles, which abbreviates common English words like “North” and “Street”. Abbreviating the road name in the turn banner will improve consistency with this label.

This Swift 2.3 file extends String with a method that returns an abbreviation that helps the string fit better given a width and font.

The list of abbreviations should be localizable. Localizers may have to take care not to abbreviate words that would be abbreviated differently in other languages.

/cc @bsudekum @cammace