Added support for getting the output from a system call

  - In JavaScript the user can now make a call to IO.systemCall()
    and a string will be returned with the output from that call
  - Only available on linux and darwin
  - Updated library/alljoyn.js as well as documentation to reflect these changes

Change-Id: I4af9fa12c51caf607fed28be2ea8b1e085e93c90
Signed-off-by: gburns <gburns@qce.qualcomm.com>
Signed-off-by: James Prestwood <jprestwo@qce.qualcomm.com>