Add bio, hireable, and twitter_username fields to Person

[Chew]

Description

** Describe your change here**
This adds 3 fields to the GHPerson abstract method
getBio() which returns the user's bio as a String
getTwitterUsername() which returns their Twitter username as a String
isHireable() which returns if they're hirable as a boolean. (The API itself says "hireable", I am just going off of what that says)

These all appear for People and Organizations as well.

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[bitwiseman]

https://developer.github.com/v3/orgs/#update-an-organization

Looks like organization doesn't have hireable or bio. So those should be moved to GHUser.

Then please add test(s) that include reading each of these fields from somewhere in a non-null/or non-false form.

[Chew]

Oh I see, if I do /users/:org it shows up, but if I do /orgs/:org it doesn't show up. Why is the API so confusing? I'll move it over!

[bitwiseman]

@Chew
And finally a user or org with non-null twitter_username.

[Chew]

I changed it to my profile since it

1. had a not null bio
2. had a not null twitter username
3. hireable is true