Calling v8::Function::Call directly for async callbacks leads to very
odd behavior in some circumstances, specifically the next tick of the
event loop seems to get stuck. After much digging, I found references to
this being a "known" issue, and the proper way to call async functions
in JS is to use the node::MakeCallback helper, or the Nan equivalent. In
some cases, we also use the Nan::Callback object as a helper for
managing JS callbacks.

KNOWN ISSUES: On node <v6, it seems that exceptions thrown from JS when
called from .NET do NOT propagate back to .NET correctly. Instead, they
terminate the JS process. This appears to be a known limitation of using
the MakeCallback method, with no easy workarounds. On 6.4.0, this test
does pass, which indicates the issue has been fixed in node.

The previous fixes to use Nan::MakeCallback had the side effect of
swallowing exceptions on Node <v6, which was a regression. To retain
that behavior, we revert the JS execution calls to using Nan::Call
instead of Nan::MakeCallback, and instead explicitly kick the next tick
with a no-op function.