Added -k option to curl command
Fixes curl failing trying to verify coursera certificate
Added netrc path as user option
Use --netrc with path. If no path is given or --n, -c and -u are omitted
a default netrc path is tried ($HOME/.netrc on *nix* or
%USERPROFILE%\_netrc on windows).
Changed logging level for several log messages
- changed logging of resource urls from info to debug
- added 'downloading' and 'already downloaded' info messages
Changed logging formats to some more readable format

- Debug format: levelname[funcname]: message
- Error format: levelname: message
- Info format: message
Curl output now as progress bar incl. percentage done
Merge pull request #53 from sdbakker/master

* Unify the handling of netrc files.
* Adjust the log level of some messages.
* Prettify curl's output.