Fix http_thrift demo to actually work for thrift over http
More counter-related refactoring

* Move _NameHelper metaclass to sparts.py
* Make a new parent class _SpartsObject for VTask and VService to inherit
* _SpartsObject.__new__() builds a .counters attribute on the generated
instance mapping heirarchical counter names to callables that return the
counter values.  Works for simple counter() and samples()
* Support heirarchical counter structures via _SpartsObject.getChildren()
* VService returns tasks via getChildren()
Comments and some whitespace cleanup
Minor TornadoHTTPTask improvements

- Add a simple `requests` (total) counter
- Add an overridable tornadoRequestLog() on TornadoHTTPTask
(default increments self.requests)
Connect FB303 counter implementation to VService counters
Update changelog