Make operator bool() explicit

This prevents unwanted conversions to bool or int such as:
```
py::object my_object;

std::cout << my_object << std::endl; // compiles and prints 0 or 1
int n = my_object; // compiles and is nonsense
```

With `explicit operator bool()` the above cases become compiler errors.

Author: dean0x7d

include/pybind11/pytypes.h

@@ -88,7 +88,7 @@ class handle : public detail::object_api<handle> {
     const handle& dec_ref() const { Py_XDECREF(m_ptr); return *this; }
 
     template <typename T> T cast() const;
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
     bool operator==(const handle &h) const { return m_ptr == h.m_ptr; }
     bool operator!=(const handle &h) const { return m_ptr != h.m_ptr; }
     bool check() const { return m_ptr != nullptr; }