Remove a gist based on it's id.

Every gist is associated to an id which also allows to remove it.
Implement the method to do so.

REF #4
Merge pull request #5 from MaximeD/gist-deletion

Remove a gist based on it's id.
REFACTOR errors handling

No need to use an else in most `if err != nil` blocks.
Simplify code where it uses it.

REF #6
Merge pull request #7 from MaximeD/refactor-switches-on-errors

REFACTOR errors handling
merged