Add a method to check Python exception types (#772)

This commit adds `error_already_set::matches()` convenience method to
check if the exception trapped by `error_already_set` matches a given
Python exception type. This will address #700 by providing a less
verbose way to check exceptions.

include/pybind11/common.h

@@ -597,6 +597,9 @@ public:
     /// Clear the held Python error state (the C++ `what()` message remains intact)
     void clear() { restore(); PyErr_Clear(); }
 
+    /// Check if the trapped exception matches a given Python exception class
+    bool matches(PyObject *ex) const { return PyErr_GivenExceptionMatches(ex, type); }
+
 private:
     PyObject *type, *value, *trace;
 };