[c++0x] <date_time> implementation, take 3

[Paolo Carlini]

Hi Pedro,
> The test suite hasn't changed since take 2.
... but remember to use -std=gnu++0x in all the tests.

Besides that, I think we are almost ready for the commit, only, please 
double check the uglifications: I'm seeing locals with names such as ns 
and xns in some operators... Also, as a largely stylistic issue, please 
fully qualify std::time, std::time_t.

Paolo.