[README.md] Fix typo in OAuth example

The success handler's access token is defined as an optional value in the implementation.
```swift
public typealias TokenSuccessHandler = (accessToken: SwifterCredential.OAuthAccessToken?, response: NSURLResponse) -> Void
```
Merge pull request #24 from ayanonagon/patch-1

[README.md] Fix typo in OAuth example
updated for Xcode6-Beta6
Merge pull request #26 from jpsim/master

Updated for Xcode6-Beta6
Fixed method used to get closest trends.
Fix getFriendsListWithID and getFriendsListWithScreenName path
Updated for Xcode6-Beta7
Fixed getStatusesShowWithID: setting "id" parameter.
Merge pull request #30 from tsuchikazu/fix_friends_list_path

Fix getFriendsListWithID and getFriendsListWithScreenName method path
Merge pull request #31 from tsuchikazu/update_for_xcode6_beta7

Updated for Xcode6-Beta7
Merge pull request #32 from gecko655/master

Fixed getStatusesShowWithID: setting "id" parameter.
Merge pull request #28 from PiotrSzmyt/master

Fixed method used to get closest trends (getTrendsClosestWithLat)
Fixed issue #34

Clue of this commit is here: http://stackoverflow.com/questions/24099520/commonhmac-in-swift
Merge pull request #35 from gecko655/SHA1DigestWithKey

Fixed crash when generating SHA1 digests, fixes #34
Change + to +| for Dictionary

Fixes issue where + operator was breaking for normal math in file that
the framework was imported into.
Make access token properties public, fixes #41