Renamed gist delete method

Renamed gist `delete` method to `remove` because it's a reserved word
(that will cause a parsing error in air applications and older
browsers) and to be consistent (the method to delete a file in a repo
is also named as `remove`.

Parsing error from air application:
![bildschirmfoto 2013-06-11 um 13 02 39](https://f.cloud.github.com/assets/2519814/636527/7816785c-d286-11e2-9c42-c7a62360a30c.png)