Adds automatic casting on assignment of non-pyobject types (#551)

This adds automatic casting when assigning to python types like dict,
list, and attributes.  Instead of:

    dict["key"] = py::cast(val);
    m.attr("foo") = py::cast(true);
    list.append(py::cast(42));

you can now simply write:

    dict["key"] = val;
    m.attr("foo") = true;
    list.append(42);

Casts needing extra parameters (e.g. for a non-default rvp) still
require the py::cast() call. set::add() is also supported.

All usage is channeled through a SFINAE implementation which either just returns or casts. 

Combined non-converting handle and autocasting template methods via a
helper method that either just returns (handle) or casts (C++ type).

tests/pybind11_tests.cpp

@@ -39,7 +39,7 @@ PYBIND11_PLUGIN(pybind11_tests) {
     for (const auto &initializer : initializers())
         initializer(m);
 
-    if (!py::hasattr(m, "have_eigen")) m.attr("have_eigen") = py::cast(false);
+    if (!py::hasattr(m, "have_eigen")) m.attr("have_eigen") = false;
 
     return m.ptr();
 }