Make operator bool() explicit

[dean0x7d]

This prevents unwanted conversions of py::object to bool or int:

py::object my_object;

std::cout << my_object << std::endl; // compiles and prints 0 or 1
int n = my_object; // compiles because of secondary implicit conversion 
                   // of `bool` to `int` <-- nonsense

With explicit operator bool() the above cases become compiler errors. The output streams will only compile when pybind11/stl.h is included, as intended. if statements and logical expressions are considered explicit conversions, so they work as intended.

[wjakob]

Great, thank you!