Add a method to check Python exception types

[romanvm]

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:

py::dict val;
try {
  val = d["key"];
} catch (py::error_already_set& exc) {
  if (exc.matches(PyExc_KeyError)) {
    exc.clear();
    return;
  } else {
    throw;
  }
}

[dean0x7d]

Looks pretty convenient! The only suggestion I have is to turn the example code into a test and add it to tests/test_exceptions.cpp.

[romanvm]

Done.

[wjakob]

LGTM

[dean0x7d]

Merged. Thanks @romanvm!