Python3

I needed to use your Python3 version of oauth2client but I got into two issues which gave me errors. So I decided to fix them:
  * `httplib2.Http`'s `request()` method in python3 returns the `content` as `bytes` instead of `str` (as it was in python2) which caused `_parse_exchange_token_response(content)` to fail because the inner json parsing machinery checks if the `content` is of type `str`. To fix this I simply converted it to `str` using utf-8.
  * `_urlsafe_b64decode()` function wrongly concatenated `bytes` (which came from calling `encode()` on the `b64string` argument) with a `str` (created by multiplying the string `"="`). I fixed this by concatenating `bytes` and at the end converting to `str` using utf-8.