Bugfix pub traits

Previously this would fail to build with the error:

    dougs-Mac-mini:rust-cocoa doug$ cargo build
       Compiling cocoa v0.1.0 (file:///Users/doug/dev/rust-cocoa)
    /Users/doug/dev/rust-cocoa/src/base.rs:77:40: 77:41 error: private type in exported type parameter bound
    /Users/doug/dev/rust-cocoa/src/base.rs:77     unsafe fn send_bool<S:ObjCSelector,A:ObjCMethodBoolArgs>(self, selector: S, args: A) -> bool;
                                                                                     ^
    /Users/doug/dev/rust-cocoa/src/base.rs:78:41: 78:42 error: private type in exported type parameter bound
    /Users/doug/dev/rust-cocoa/src/base.rs:78     unsafe fn send_point<S:ObjCSelector,A:ObjCMethodPointArgs>(self, selector: S, args: A) -> NSPoint;
                                                                                      ^
    error: aborting due to 2 previous errors
    Could not compile `cocoa`.

NB. This doesn't fix the doc test failures, which are related to trait visibility.