direct_client gen_headers is inconsistent

consumers of direct_client should be able to reliably over-ride the user-agent header and set x-timestamp

We should replace gen_headers with a more useful utility and make sure all direct_client methods accept the headers kwarg

Something like this:

    def prepare_headers(headers, ensure_ts=True):
        """
        Get the headers ready for a request. All requests should have a User-Agent
        string, but if one is passed in don't over-write it. Not all requests will
        need an X-Timestamp, but if one is passed in do not over-write it.

        :param headers: dict or None, base for HTTP headers
        :param ensure_ts: boolean, should be True for any "unsafe" HTTP request

        :returns: HeaderKeyDict based on headers and ready for the request
        """
        hdrs_out = HeaderKeyDict(headers) if headers else HeaderKeyDict()
        if ensure_ts and 'x-timestamp' not in hdrs_out:
            hdrs_out['X-Timestamp'] = Timestamp.now().internal
        if 'user-agent' not in hdrs_out:
            hdrs_out['User-Agent'] = 'direct-client %s' % os.getpid()
        return hdrs_out

... and all the tests that go with it