Allow enabling self-signed SSL certificates at runtime

Currently, the ability to use self-signed SSL certificates is enabled through a preprocessor macro. This means the app can't decide at runtime whether to allow these certificates. This pull requests adds a property to AFHTTPClient called allowInvalidSSLCertificate, which when set to YES allows invalid SSL certificates. To maintain backwards compatibility, the existing preprocessor macro is honoured.

Although obviously not a good idea to enable in a public build of an app, during testing we choose which server to use at runtime and we need to authenticate against a server that uses a self-signed certificate. This greatly helps with our ability to work against this server.