Remove keychain from canvaskit

We really shouldn't be asserting opinions about the keychain
in our networking library. Clients can handle this on their own
if they want.
Merge remote-tracking branch 'instructure/CanvasKit/master'
Add logout functionality

lets the client tell the server to reject the
current access token.
update readme
Make logout set accessToken and user to nil.
remove clientID from header
Add private currentUser property
stop retaining modal web view

this caused bugs if you would ever logout and try logging back in again.
Merge pull request #48 from jarsen/master

Remove Keychain Schema + More