Add test for angle_v2_rad()
Don't catch overflow

It's probably better for the error to be visible so the problem
can be seen and addressed.
May readd this try/except if the exception is too common, but ideally
this will be a rare exception.
Disable Vec2.add() since += is available

__iadd__ and += are about the same, but they don't let you use
a return value.  This should not be a problem for known use cases.
Remove catching of Overflow errors.

Similar to the change a couple commits before this, keep the error
visible so it can be found and corrected.
Add test to ensure vec_a * vec_b raises exception
Merge pull request #12 from olsoneric/vec2_tests

Vec2 tests