Continous Integration + Interop Tests (on java, python, c#) + Attached Signature fix

Instead of an environment variable for test data, pulling in a git submodule of testdata, allowed configuring for travis-ci support. 

Added suite of unit tests against java, python, c# data, but had to skip TimeoutVerifier tests as I didn't know a good way to swap out the current time.

Fixed an issue found by the tests, in which a nil nonce doesn't produce the attached signature corretly

http://code.google.com/p/keyczar/source/browse/java/code/src/org/keyczar/Signer.java#223
```java
    // Attached signature signs:
    // [blob | hidden.length | hidden | format] or [blob | 0 | format]
    byte[] hiddenPlusLength = Util.fromInt(0);
    if (hidden.length > 0) {
        hiddenPlusLength = Util.lenPrefix(hidden);
    }
```