Added upload_image functionality

[kn0wm4d]

What does this PR do?

Added upload_image functionality for both posts and stories

from PIL import Image
from instagram_private_api import Client

api = Client(username=...,password=...)

with Image.open('/Users/kn0wm4d/Desktop/test.jpg') as img:
   photo_data = open('/Users/kn0wm4d/Desktop/test.jpg', 'rb')
   width, height = img.size
   size = (width, height)

api.upload_image(photo_data, quality=80, size=size, caption='Test', story=False)

Why was this PR needed?

Because IG deprecated post_photo (experimental endpoint).

What are the relevant issue numbers?

#305

Does this PR meet the acceptance criteria?

• [x] Passes flake8 (refer to .travis.yml)
• [x] Docs are buildable
• [x] Branch has no merge conflicts with master
• [ ] Is covered by a test

[MrTob]

the upload_image functionality returns a 500 HTTP Error

urllib.error.HTTPError: HTTP Error 500: Internal Server Error