Merge pull request #19 from rjw57/test-downloader

Some unit tests! Testing the downloader.
tests: move TemporaryDirectoryTestCase to util module
tests: move responses_add_mocks to tests.util
dataset: support configurable tile resolution

This is a requirement for being able to test the dataset code with
non-full resolution tiles.
test: change mock data tile size to 20x10

The input tiles are non-square and non-power-of-two in size. Reflect
this in the mock data.

Update the generate-dem-chunks.sh script to also save a ruaumoko-format
dataset for use in later testing.
tests:dataset: add stub tests for Dataset

These tests do little more than check that the dataset can be opened and
that the mock dataset location exists.
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.
setup: add GDAL dependency

GDAL is a library which allows direct access to a variety of Geospatial
Data formats. (It is, in fact, a Geospatial Dataset Abstraction
Library.)

The Python bindings are distributed separately on PyPI as the GDAL egg.
Add it as a requirement to setup.py.
travis: add GDAL setup magic

On the 12.04 Ubuntu boxes which act as Travis workers, the distro GDAL
packages are quite old so add the ubuntugis PPA to enable a more modern
GDAL to be installed. Also specify the exact version of the Python
bindings to install in order to match the installed GDAL.