[Shared Runtime Process Model] Class skeleton added.

According to the discussion and the new design for the shared runtime
process model, the previous pull request
(https://github.com/crosswalk-project/crosswalk/pull/750)
is abandoned. This patch is the first step towards the new design goal.
A new switch --daemon is added to let xwalk go into daemon mode.
Loading a simple web page from command line is still okay.
Please refer to
(https://docs.google.com/a/intel.com/document/d/1q2Zl_P949NLe5YnQoLP2OWcZStVJgXyHKZDUwhiUxJQ/edit#)
for more design details.