Fix/issue345 - #347

Fixes three issues:

1. python27 doesn't have a bytes() method that's compatible with python3's bytes() method, so I had to change to "a string".encode('utf8') for constructing the body of a chunked upload (i.e., a GIF or MP4).

2. The UploadMediaChunked() method was pretty much untestable in its previous form and did way too much within a single function, so it is now broken out into three functions:

   a. _UploadMediaChunkedInit() <- starts the upload
   b. _UploadMediaChunkedAppend() <- sends actual data
   c. _UploadMediaChunkedFinalize() <- finishes the upload

   These get called in sequence in UploadMediaChunked() and that method is called and returns as before, but now we can test the individual parts of the upload.

3. Calling PostUpdate() tried to iterate over an int if you called it with media=fp, so that got fixed too.

This change is