[download] Add exception handling

This commit added exception handling and bug fixing according to
tct-download-tizen-tests.
Merge pull request #144 from deqing/fix-tizen-exception

[download] Add exception handling
[SystemInfo] Use singleton model to support multi frames

The old design is one frame one set of listening mechanism. So when there are multi-frames,
there will be multiple sets of events. However, the vconf only support to register the first one
when asking twice for the same vconf_notify_key_changed with same Key and vconf_callback_fn
and the others will be ignored. So, the old framework couldn't resolve the vconf issue.

The current design uses singleton event listening mechanism.
Every property object (such as: cpu, local, storage ...) is singleton.
Each of them will maintain an according events list. Use StartListening(Context* api) to
push back the current context to the according list. Use StopListening(Context* api) to
remove the current context from the list. When the property value changed,
iterate the list to post message to all the listenng context.
Merge pull request #155 from qjia7/master

[SystemInfo] Use singleton model to support multi frames
[Time] Add getAvailableTimezones
[Time] Fix TZDate constructor

Both date and timezone are optional.

Now supporting the following example:

foo = new tizen.TZDate(null, "Europe/Berlin");
[Time] Change the way of construction of TZDate

This will reduce the possibility of mistakes between things
like getDate() and getDay().
[Time] addDuration is now throwing exceptions
[Time] Add the checks for timezone property
[Time] Solve instanceof problem
[Time] Fix timezone -> timezone_ mistype
[Time] Fix unecessary check
[Time] Properly format toDateString, toTimeString and toDateTimeString