Add Followers API

[obsc]

@pengwynn

Added functionality for https://developer.github.com/v3/users/followers/
Also added booleans as an additional return just as we discussed.

[pengwynn]

@obsc I dig the boolean returns. How would you feel about updating it to match the new approach we're pioneering over in #85 where each method takes a hyperlink instead of having a field on the service?

[obsc]

@pengwynn Refactored to the new approach in #85.

One issue that might need addressing is

if err != nil {
    return nil, &Result{Err: err}
}

is never called upon testing atm. Should I add failure tests that try to expand a broken hyperlink to hit 100% test coverage?

[pengwynn]

Should I add failure tests that try to expand a broken hyperlink to hit 100% test coverage?

That sounds awesome and would make @dannysperling really happy.

[obsc]

@pengwynn Fixed tests to include a failure test now.

[pengwynn]

✨ Thanks!