added QUnit.precision existence check in number comparison to handle floats

Strict comparison of numbers is making tests unstable if they are validating calculation results.
In single cases it is possible to write 
ok(2.6 - 2.59999999996 < 1e-6);
but is case of arrays or objects containing floats the above is not going to work

The change is adding precision support into QUnit.
The default behavior is not changed