Issue #134: add a bonobo download url command

This enables users on different platforms to download the examples in
the tutorial using the same command.

tests/test_commands.py

@@ -13,6 +13,7 @@ from cookiecutter.exceptions import OutputDirExistsException
 from bonobo import __main__, __version__, get_examples_path
 from bonobo.commands import entrypoint
 from bonobo.commands.run import DEFAULT_GRAPH_FILENAMES
+from bonobo.commands.download import EXAMPLES_BASE_URL
 
 
 def runner(f):
@@ -152,6 +153,29 @@ def test_version(runner):
     assert __version__ in out
 
 
+@all_runners
+def test_download_works_for_examples(runner):
+    fout = io.BytesIO()
+    fout.close = lambda: None
+
+    expected_bytes = b'hello world'
+    with patch('bonobo.commands.download._open_url') as mock_open_url, \
+            patch('bonobo.commands.download.open') as mock_open:
+        mock_open_url.return_value = io.BytesIO(expected_bytes)
+        mock_open.return_value = fout
+        runner('download', 'examples/datasets/coffeeshops.txt')
+    expected_url = EXAMPLES_BASE_URL + 'datasets/coffeeshops.txt'
+    mock_open_url.assert_called_once_with(expected_url)
+
+    assert fout.getvalue() == expected_bytes
+
+
+@all_runners
+def test_download_fails_non_example(runner):
+    with pytest.raises(ValueError):
+        runner('download', '/something/entirely/different.txt')
+
+
 @all_runners
 class TestDefaultEnvFile(object):
     def test_run_file_with_default_env_file(self, runner):