Allows the creation of custom prompts and responses to allow custom prompts for contextmanagers.

Allows the creation of custom prompts and responses. I wrote it to allow custom contextmanagers as in the following example:

``` python
from fabric.api import env, run
import contextlib

@contextlib.contextmanager
def asu(user, asu_number):
    '""custom ASU wrapper around sudo"""

    save_shell = env.shell

    save_current_prompts = env.prompt_responses
    env.prompt_responses = { "Enter ASU #:" : asu_number, "Password:" : "testpassword" }

    env.shell = '$HOME/asu  %s -c' % user
    yield
    env.shell = save_shell
    env.prompt_responses = save_current_prompts
```

And in the fab file:

```
with asu("root",asu_number):
      run("touch /var/tmp/test.touch")
```