Clean up urls

Add CLASS_URL, AUTH_REDIRECT_URL.
Drop the email and visiting query arguments in the authentication
redirector url.
Remove unnecessary preview arguments.
Rename className to class_name

Be consistent.
Add requests dependency
Add tests that check if we can load cookies files
Add find_cookies_for_class

Instead of the global variables csrftoken and session, we are going to
store the cookies in a requests session. The function
find_cookies_for_class searches for cookies in a given cookies_file and
returns a RequestCookieJar. This jar will be given to the requests
session.
Add do_we_have_enough_cookies

This replaces the extract_session_and_csrftoken_from_cookiejar function
Add make_cookie_values

This function will be used to create the cookie header needed for the
external downloaders.
Use requests session in down_the_wabbit_hole
Add get_authentication_cookies

A cookies file should contain the cookies for ww.coursera.org. If it
contains the cookies for class.coursera.org we will use them, otherwise
we get them via down_the_wabbit_hole. The get_authentication_cookies
does this. It replaces the set_session_and_csrftoken function.
Add login function

We store the cookies in a cookiejar instead of writing them to a
tempory file. The login function repacles the write_cookie_file.
Use requests session in get_page and get_syllabus
Use requests session for downloading
And its gone
Add session to parse_syllabus, grab_video_url and get_video

Sometimes when we the lectures page, we have to make some extra
requests. So we have to make the session available to these functions.
PEP-8
Fix make_cookie_values

The cookies should be seperated by a semicolon, not a comma.
Apparently, this is only problematic for mp4 files …
Remove unused import
Merge pull request #120 from jonasdt/with_requests

Add Jonas's work to use the requests library and remove a lot of cruft on the way.