no namespace-level using statements

[Bob Carpenter]

Please don't include namespace-level includes.

The problem is that then you can't control locally what
gets included, which can lead to name-resolution conflicts,
and it also introduces order dependencies on the files.

See: http://stackoverflow.com/questions/6175705/scope-of-using-declaration-within-a-namespace

or go straight to Herb Sutter's advice:

http://www.gotw.ca/gotw/053.htm

That site's awesome, by the way, if you want to study C++ :-)

- Bob