Refactor code generation and API definition
Make everything updatable (temporarily)
Mark Repository structures as not updatable

.AnonymousContributor is returned only by .get_contributors
.Branch has no url, and _link["self"] if not returned by .get_branches
.ContentCommit is ephemeral (hence has not url)
.Tag is returned only by .get_tags
Todo
Mark Repository structures as not updatable

.GitIgnoreTemplate could be updatable and completable if we returned it from get_git_ignore_templates
.Meta could be updatable with a constant url
.RateLimit and .Resources could be updatable with a constant url
Keep parameters order from api doc

Why did I sort them in the first place?!
Check unexisting parameters
Refactor parameters with an origin (like User.login)
Some things about Issues (#228)