Open cookies file in universal newline mode

In the cookies files exported with the firefox addon on a mac, every
line is terminated with a carriage return \r. Loading this file into a
MozillaCookieJar has the unwanted effect of having every cookie value
ending with \r. With the universal newline mode enabled, the \r is
replaced with a \n.
Monkey patch cookielib.Cookie.__init__

The max-age value in a cookie can be a decimal, e.g., in the file
exported with the chrome extension (this is allowed by the rfc). The
cookielib.Cookie class on the other hand is only compatible with
integer values. This is unacceptable behaviour and should be punished.
Fix downloading when authenticated via cookies file

The global variable csrftoken was not set when a cookies file was used.
Hence downloading files did not work. Authentication is now split into
two stages:
- get the login cookies from www.coursera.org. If a cookies file is
used, it should already contain these cookies.
- get the session and csrf_token cookies from  class.coursera.org. If a
cookies file is used, we first check if we already have the cookies,
otherwise we retrieve them as usual.
Update README

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