Simplified unix epoch time

Yes, this is on the way :)

any ideas for this, Steve?

After some thought, there doesn't seem to be a real compelling reason to have a separate tango-only function that does the equivalent of time(null). I think keeping the API clean and having only one time base overrides having the convenience and support of Unix time also. We probably would then have to add Windows time.

I think the current method of either using time(null) or calculating by using the epoch constants is sufficient for most people's needs.

That being said, I did add to the documentation methods on how to calculate unix time from tango time, which should be beneficial to users who must work with unix time from other libraries.

(In [3167]) Added more documentation to help explain how Tango time relates to Unix time (references #763)