add the possibility of showing the stacktrace when an exeption in raise in the sandboxed code

the use is as follow

```python
def f():
    x
from sandbox import Sandbox, SandboxConfig,SandboxError
sandbox = Sandbox(SandboxConfig('stderr',use_subprocess=True))
try:
    result = sandbox.call(f)
except SandboxError as e:
    if hasattr(e,'stacktrace'):
        print (e.stacktrace)
    else:
        raise
```

moreover the modifications remove a security issue as the Exception was previously pickled and could be execute untrested code when unpickled