[Hitbtc] To be consistent with other implementations, HitBit nonces are ...

...generated from system milliseconds
toString() is a quick fix of Bug in si.mazi.rescu library. If we use @QueryParam the nonce value provided by ValueFactory is obtained with the use of toString() method instead of cerateValue():

Line in class RestInvocation:
String queryString = paramsMap.get(QueryParam.class).asQueryString();