implement __geo_interface__ for Point, Line, Polygon

Note: there is an imperfect mapping between guppy types and those in Sean
Gillies __geo_interface__ proposal. guppy types are generally more like Features
than Geometries, but here I've treated them as the latter. This will make it
easy to pass guppy objects to libraries like shapely, but loses the properties
and metadata associated with each object.

Also, since __geo_interface__ is stored as an attribute, I don't provide bbox
because keeping this up to date would have performance implications. using the
property decorator might be a good workaround.

reimplement __geo_interface__ using properties

mention __geo_interface__ in readme