src: reorganize process object creation, property implementation and events in C++ land

[joyeecheung]

src: move process object creation into node_process_object.cc

Changes SetupProcessObject to CreateProessObject which creates
the process object from scratch and return it to Environment::Start
to be stored in the Environment object.

src: declare process-related C++ methods in node_process.h

Instead of in node_internals.h. Also move process property
accessors that are not reused into node_process_object.cc
and make them static.

process: move C++ process events into node_process_events.cc

Move the C++ process.emit and process.emitWarning methods
from node.cc into into node_process_events.cc, and
reuse the implementation in other places that need to do
process.emit in C++.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines