Fixed #831: Arithmetics counter fails if only one parameter is given
Fixed propagation of hpx_main's return value to hpx::init()
Constructing a future from a wrapped future is no longer an extension

Signed-off-by: K-ballo <k@fusionfenix.com>
Implicit unwrapping (one level) of future<future<R>> to future<R> in then().

See Issue #828