print traceback on failed import

[esquires]

This PR prints a traceback when a python import fails due to an error in the imported file. The existing implementation will print out a message about the module not being found. Here is an example demonstrating the issue:

// example.cpp
#include <Python.h>
#include <pybind11/pybind11.h>

int main(int argc, char *argv[]) {
    Py_Initialize();
    pybind11::module::import(argv[1]);
    Py_Finalize();
    return 0;
}

# example.py
raise Exception('exception raised on import')

Before this PR:

build: ./ex example                                                                     
terminate called after throwing an instance of 'pybind11::import_error'
  what():  Module "example" not found!
[1]    3744 abort (core dumped)  ./ex example
build: ./ex module_that_doesnt_exist                                                   
terminate called after throwing an instance of 'pybind11::import_error'
  what():  Module "module_that_doesnt_exist" not found!
[1]    3766 abort (core dumped)  ./ex module_that_doesnt_exist

After this PR:

build: ./ex example                                                                 
terminate called after throwing an instance of 'pybind11::error_already_set'
  what():  Exception: exception raised on import

At:
  /home/esquires3/Documents/pybind/example.py(1): <module>

[1]    3650 abort (core dumped)  ./ex example
build: ./ex module_that_doesnt_exist                                                      
terminate called after throwing an instance of 'pybind11::error_already_set'
  what():  ImportError: No module named module_that_doesnt_exist
[1]    3664 abort (core dumped)  ./ex module_that_doesnt_exist

[wjakob]

That looks good to me -- thanks!