Dont sign xAuth request with (potentially invalid) token or token secret

# TMTumblrAuthenticator

* Fix a bug where state remaining from a previous login could result in a subsequent login request being signed with invalid tokens
    * Remove as much state as possible from this class
        * No more `OAuthToken` property, this is never needed outside of individual methods
        * Request signing method is now a class method that takes all input via arguments, rather than references properties
        * `OAuthTokenSecret` property renamed `ThreeLeggedOAuthTokenSecret` to make sure it isn't accidentally use for xAuth
* Added curly-braces around all single line conditionals (post-`goto fail;` stress)
* Replaced occurrences of `int` with `NSInteger`

## AuthenticationExample

* Fixed bug in example URL scheme

## project.pbxproj

* Fixed debug compilation in Xcode 5