api: Support file uploads to the API.

Now, the `Client.do_api_query()` method supports sending files to the
API.

This has allowed the implementation of a new method,
`Client.upload_file(file)`.  It simply uploads the file set in the
parameter, and returns the API's response (that includes the URI).

Despite the fact that `do_api_query()` supports multiple files as
parameters, `upload_file()` doesn't, because right now the API isn't
capable of managing more than a file in the same request.

examples/upload-file

@@ -0,0 +1,66 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+# Copyright © 2012-2017 Zulip, Inc.
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+from __future__ import print_function
+import importlib
+import optparse
+import sys
+
+from six.moves import StringIO as _StringIO
+sys.path.insert(0, './api')
+from typing import IO
+import zulip
+
+class StringIO(_StringIO):
+    name = '' # https://github.com/python/typeshed/issues/598
+
+usage = """upload-file --user=<user's email address> --api-key=<user's api key> [options]
+
+Upload a file, and print the corresponding URI.
+
+Example: upload-file --user=cordelia@zulip.com --api-key=a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5 --file-path=cat.png
+
+You can omit --user and --api-key arguments if you have a properly set up ~/.zuliprc
+If no --file-path is specified, a placeholder text file will be used instead.
+"""
+
+parser = optparse.OptionParser(usage=usage)
+parser.add_option('--file-path')
+parser.add_option_group(zulip.generate_option_group(parser))
+(options, args) = parser.parse_args()
+
+client = zulip.init_from_options(options)
+
+file = None # type: IO
+if options.file_path:
+    file = open(options.file_path, 'rb')
+else:
+    file = StringIO('This is a test file.')
+    file.name = 'test.txt'
+
+response = client.upload_file(file)
+
+try:
+    print('File URI: {}'.format(response['uri']))
+except KeyError:
+    print('Error! API response was: {}'.format(response))