feature(api): Change u64 ids to &str

Breaking change: All the entities with an `id` property have String ids, but all
endpoints that take ids used `u64` ids. This changes that so that all
the methods that take ids, take them as `&str`.

examples/follow_profile.rs

@@ -9,7 +9,7 @@ use std::error;
 fn main() -> Result<(), Box<error::Error>> {
     let mastodon = register::get_mastodon_data()?;
     let input = register::read_line("Enter the account id you'd like to follow: ")?;
-    let new_follow = mastodon.follow(input.trim().parse()?)?;
+    let new_follow = mastodon.follow(input.trim())?;
 
     println!("{:#?}", new_follow);