Removed USGS retrieval code.

The project no longer automatically retrieves files from the USGS due
to changes in their web services.  A workaround has been configured
which requires the user to download their own data from the USGS using
the National Map Viewer.  Once that data has been retrieved, the
software can use that data to create Minecraft worlds.

Of course, the worlds don't work with current Mojang code due to the
biome changes, but that issue will be solved wiht another branch.

Still need to make multiple input files work.  Once that's done, this
will be merged into develop.
Now caching extracted maps in downloads directory.

Users still need to specify the downloaded maps on the command line,
though, even if the maps have already been downloaded.
Added support for multiple files, updated documentation.

The README.md got cleaned up and updated to reflect changes in the code.

region.py now supports lists of files for elevation and land cover as
well as individual files.

getregion.py changed to reflect the changes in region.py.
Merge pull request #47 from mathuin/no-more-downloading

No more downloading, in the right branch this time!