test_pytumblr.py: ensure tumblr_credentials.json gets closed

Put the read in a with statement to close the file when we're done
with it.
TumblrRestClient.send_api_request: ensure files get closed
TumblrRestClient.create_text: reject invalid 'text' kwarg
TumblrRestClient.reblog: add 'comment' param to docstring
TumblrRestClient.edit_post: specify valid options

Pass valid options to send_api_request, adding missing 'id' parameter.