Updated slice's signature.

Changed 'view' to 'viewport' to be more consistent with the view function, making direct argument passthrough more transparent.
Added the slice function.

The slice function is the core of the stiching functionality. It will yield chunks of the map for iterative processing.
Updated tests to use the new directory structure. Added test_vis.py.
Now the scale argument to generate_map DOES something.
Now test_vis makes sure that chunked DataFrames have the right shape.
The chunk function now has tests and is ready for integration into the stitching project.
Changed the signature of view() and chunk().

I wish I could remember why I had x and y transposed in the viewport argument, but I don't. Tests pass with x and y in the logical order, so I'm committing to it.
Tests were running slowly.

I halved the test suite's map size because one and a half seconds for the test suite was too slow.
Created demo.py

I moved the demo function out of this file because it's going to become the main entry point for interactive use of Genesis.
Renamed demo.py

demo.py is now genesis.py, the main entry point for interactive use of Genesis.
genesis.py will only run the demo when asked now.
The demo is now working as it should.
Updated the README.
Updated command line arguments for genesis.py.

I added a help message to all the arguments and used argparse's argument groups to make things more sensible.
Small docstring tweaks.
Spacing tweak.
Docstring tweak.
Small tweak to the scaling value.
Merge branch 'stitches'
This is a merge to consolidate work so that I can being working on the pypy feature branch.
Unpinning version requirements. For now?