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.

bonobo/commands/download.py

@@ -0,0 +1,43 @@
+import io
+import re
+import urllib.request
+
+import bonobo
+
+EXAMPLES_BASE_URL = 'https://raw.githubusercontent.com/python-bonobo/bonobo/master/bonobo/examples/'
+"""The URL to our git repository, in raw mode."""
+
+
+def _save_stream(fin, fout):
+    """Read the input stream and write it to the output stream block-by-block."""
+    while True:
+        data = fin.read(io.DEFAULT_BUFFER_SIZE)
+        if data:
+            fout.write(data)
+        else:
+            break
+
+
+def _open_url(url):
+    """Open a HTTP connection to the URL and return a file-like object."""
+    response = urllib.request.urlopen(url)
+    if response.getcode() != 200:
+        raise IOError('unable to download {}, HTTP {}'.format(url, response.getcode()))
+    return response
+
+
+def execute(path, *args, **kwargs):
+    path = path.lstrip('/')
+    if not path.startswith('examples'):
+        raise ValueError('download command currently supports examples only')
+    examples_path = re.sub('^examples/', '', path)
+    output_path = bonobo.get_examples_path(examples_path)
+    fin = _open_url(EXAMPLES_BASE_URL + examples_path)
+    with open(output_path, 'wb') as fout:
+        _save_stream(fin, fout)
+    print('saved to {}'.format(output_path))
+
+
+def register(parser):
+    parser.add_argument('path', help='The relative path of the thing to download.')
+    return execute