Support OGRs network and compressed file access methods.

I.e. file names starting with /vsicurl/ and/or /vsizip/, /vsigzip/
and /vsitar/.
Handle .gz .tar.gz and .zip files transparently.
Add options --idfile and --saveid to read and write starting and ending ID from files.
Update tests for --idfile and --saveid options.
Make elementIdCounter a class variable in Geometry.

Simplify getNewID().
Make globals features and geometries class variables.
Move class definitions to a separate file, geom.py.

This is to make them accessible to translation functions.
Better support for OGR virtual file systems.

Do not call os.path.realpath() on input filename to preserve slashes in e.g.
/vsicurl/ input strings.

Divide OGR virtual file systems to three classes:

Access methods: Filenames are passed unmodified to OGR.

File methods: Existence of file is checked before passing the name to
OGR.

Unsupported methods: These do not make sense for input, emit an error.
Merge pull request #23 from tpikonen/master

OGR access methods, ID counter saving and separation of classes