Add ability to get user's display name

[anoadragon453]

Add the ability to pull down a user's displayName given their userID as defined by https://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-client-r0-profile-userid-displayname

Additionally, the ability to track the m.room.member and m.presence event that are sent on a displayName change as defined by https://matrix.org/docs/spec/client_server/r0.2.0.html#events-on-change-of-profile-information, would be useful.

[MTRNord]

First part of this Issue is being done in #37 (Even your link goes to the Avatar URL not the displayName spec)

[anoadragon453]

Whoops, corrected thanks. And awesome to see, thanks for doing that (and thanks to @t3chguy as well)!