add new tool: ruaumoko-ascii-map

This is a silly little tool which can be used to visually verify that
the dataset which has been downloaded is sane and being parsed
correctly. When run, it generates a little ASCII-art elevation map. If
the output looks like the world map then you can have some confidence
that the dataset has downloaded correctly.
tests:asciimap: add a simple test for ASCII map

Add a simple test which checks that the ASCII map output matches the
expected output when run with the mock data. This essentially verifies
that the Dataset.get() function works sanely over the full range of
input values.
tests:asciimap: fix test on Python 3

Patching print() is really rather subtle. Instead of patching print
directly, patch sys.stdio.write.
asciimap: do not use print() for output

The fact that print() uses std.stdout.write() under the covers is
probably non-portable. Don't rely on this for the tests to pass.
tests:asciimap: increase coverage

Exercise the "validate shape" function a little by passing some invalid
command line options in.
Merge pull request #21 from rjw57/ascii-map

Visual verification tool: ascii map