Put keychain access into the background

[sbeitzel]

Running a debug build of this project for the first time, I go through the sign-in flow. This ultimately calls into the method referenceForApplicationAtURL in NSBundle+KeychainAccess.m in the CoreTootin module. Internally, that method calls SecTrustedApplicationCreateFromPath which generates an Xcode warning: "This method should not be called on the main thread as it may lead to UI unresponsiveness."