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/test_eigen.cpp

@@ -40,7 +40,7 @@ test_initializer eigen([](py::module &m) {
     typedef Eigen::SparseMatrix<float, Eigen::RowMajor> SparseMatrixR;
     typedef Eigen::SparseMatrix<float> SparseMatrixC;
 
-    m.attr("have_eigen") = py::cast(true);
+    m.attr("have_eigen") = true;
 
     // Non-symmetric matrix with zero elements
     Eigen::MatrixXf mat(5, 6);