[Time] Fix TimeDuration attributes

Object.defineProperty was calling length parameter instead of
length attribute.
[Time] Truncate TimeDuration length attribute to integer

TimeDuration length attribute in IDL is long long type, it means
that we can accept only integer lengths.

We now use Math.floor() on the length attribute to truncate it.

This patch fixes the TZDate_addDuration_invalidLength TCT test.

Example:

   foo = new tizen.TimeDuration(4.5, "HOURS");

   4.5 -> 4
Merge pull request #213 from ricardotk/wip2

[Time] Truncate TimeDuration length attribute to integer