Add stub to Compose.jl interop file
Add rectangle() and polygons()

These functions convert Shapefile.jl shape primitives to their Compose.jl counterparts.

Note that Shapefile.jl polygons can be disjoint while Compose.jl polygons cannot, so the closest mapping from Polygon onto Compose.jl primitives is a list of Compose polygons, not a single polygon.
Merge pull request #3 from jiahao/with-compose

RFC: Add some conversion functions to play nice with Compose.jl