add a simple stub travis configuration

This does nothing more than attempt to install the requirements.
travis: add command for running test suite
tests: add empty test module
travis: invoke nosetests with coverage output
tests: add simple import test

Test that ``import ruaumoko`` succeeds.
gitignore: add coverage output
Merge pull request #15 from rjw57/de-duplicate-dependencies

remove duplication in requirements.txt and setup.py
Merge pull request #16 from rjw57/travis-ci

add stub support for Travis CI
download: reduce reliance on shelling out

Remove reliance on wget and unzip being installed by making use of
Python's amazingness. Specifically we use the requests library to
download the DEM in-memory and then extract the TIFF to disk. This does
require using ~50MiB of RAM in the downloader but, hey, memory is cheap
nowadays, right?