More carefully defined the concepts. That is the concept of the library is now clear:
Trap at either compile or runtime integer operations which yield a result which differs from the normal arithmetic result.
Pending on this check in:
a) only addition is fixed up so far
b) uses BOOST_TYPEOF... rather than decltype(... - final decision sill pending.
c) comments out some compile time checks which should be re-instated.
d) fixed tests so that they actually work!
