Rough first pass at limiting scrolling when zoomed in. Added a minimum
level of zoom limit for boards smaller than display
Fixed lens logic for boards that are narrower than view in one dimension
but not the other
Added missing zoom in/out bits in graphical front end and lens header
Added zoom-in/out directions in README
Fixed change log, added zoom in/out to usage string
Merge pull request #12 from calebsmith/feature/add-zoom

Added an interactive zoom feature per #11