Use the relative import syntax

This also means that the script has to be run as a package. Hence, we
replace the coursers-dl symlink with a executable script that uses
coursera_dl.main.
Add six dependency

Python 2 and 3 Compatibility Library
Renamed modules compatibility
Use UTF-8 encoding
Escape backslash

Otherwise, we have the following error:
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes
in position 845-846: truncated \UXXXXXXXX escape

http://stackoverflow.com/questions/1347791/unicode-error-unicodeescape-c
odec-cant-decode-bytes-cannot-open-text-file
Make tests independent of cookies order
Use six.iteritems

This way we always use iterators instead of lists
Use html5lib version 1.0b2

0.95 is not compatible with Python 3
Update README
Update travis config
Merge pull request #159 from jonasdt/six

Merge Jonas's work for dual compatibility with Python 2 and 3, via the `six`
module, similar to (but better than) my work in the `python3-experimental`
branch.

This closes #103.