Downloads courses with disallowed chars in name

Before fix program was unable to create directories if course name
contained invalid chars, e.g. ':' in "Stat2.1x Introduction to
Statistics: Descriptive Statistics"
Merge pull request #30 from TheRusskiy/master

Fix for issues #29 and #17, now invalid chars in directory name are being omitted
README: Add reminder to use latest version of youtube-dl.

This is always a good idea, as can be seen in issue #27.
edx-dl: Kill BOM from the beginning of the script.

The BOM breaks execution of the script as an executable under the Linux
kernel (and possibly other Unix systems as well).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
merge shk3's branch master