Switch to POST /authorizations for sign in

Add note and fingerprint fields accordingly.
Return to the PUT API, with extra fields
Refactor stubbing blocks to make status codes easy
Failing test for deleting existing authorizations
Add OCTResponse.statusCode
Handle nil response objects (e.g., on HTTP 204)
Delete any existing authorization upon HTTP 200
Accept mirage-preview for /authorizations
Fix preview Content-Type
Remove accidental logging left in
Fix typo in parameter documentation
Merge pull request #5 from github/new-authorizations-api

Switch to POST /authorizations for sign in
Only delete authorization if its token is gone

That way, older API versions (on Enterprise) that still return a token
will be unaffected.
Use an empty string for the token stub
Clarify when we won't receive a token from the API
Merge pull request #6 from github/only-revoke-missing-tokens

Only delete an authorization if its token is gone
Merge branch 'master' into upstream-master
Merge remote-tracking branch 'octokit/master'

Conflicts:
	OctoKit.xcodeproj/project.pbxproj
	OctoKitTests/OCTClientSpec.m
Fix visibility of some headers
++Mantle