Partial fix for Jira issue IOT-24.

Partial fix for Jira issue IOT-24.
    Upon a successful observe registration/dereg new return values have been added.
    These values are for success cases and need to be considered along with OC_STACK_OK.

    Patch set #2:
    1) ObserveReq struct has a member holding the result of registering and observer
    2) OCStackResult has only one success code possible

    Change-Id: I3b6fc51d64e602b8dbbca85e0e669fd7d7d9f230

Patch Set #3:
1) responded to comments
2) rebased on master

Change-Id: I2ee4d6a641b9b17adf69efca5da75cf3a4e4d3b7