A rewrite of the Gem (v0.2.1)

I rewrote the gem to make it a lot more flexible (using `base_uri`, metaprogramming for the attributes, some error handling, inheritance). I moved the `values(hsh, key)` method into the namespace & privatized it. There are now also Exercise and Video objects.

Some links where outdated and I tried to improve the readme a bit. The version number was changed to reflect the rewrite and some bug squashing.

I was also initially confused as the gem in the public repo, although being version 0.1.0, does not have some of the methods here on github (ex. get_videos/get_exercises). You may want to push the gem.

Some `#TODO` comments have been added where work is still needed (the KhanAcademy is still lacking in certain places as well). The gem still has a long way to go but it should be a lot easier to extend functionality now =)

I hope you agree with the rewrite,
/ Simon