Added support for KML input and output.

Resolves issue #3

```ruby
shp = OGR::ShpReader.new.read './spec/data/ne_110m_coastline/ne_110m_coastline.shp'
shp.to_kml '~/Desktop/github_to_kml.kml'

kml = OGR::KMLReader.new.read '~/Desktop/github_to_kml.kml'
kml.to_shp './kml.shp'
```