Message6952

I've implemented a few more of the items, as well as some 2.6 -> 2.7 changes (bug fixes, thousands separators and {}).

 - implement 'n' formatting type for integers
 - implement remaining checks for error conditions
 - limit recursion level for recursive format specs

Nearly all of the tests from 2.7 pass.  Notably still remaining from the original list:

 - implement __format__ for floats
 - implement __format__ for datetime