Add support for pdb in setup_readline

This simple change allows for Jedi completion when using pdb.  The user must have also defined a `~/.pdbrc` file with the following:

```python
import os
execfile(os.path.expanduser("~/.pdbrc.py"))
```

And a `~/.pdbrc.py` with the following:

```python
try:
    from jedi.utils import setup_readline
    setup_readline()
except ImportError:
    # Fallback to the stdlib readline completer if it is installed.
    # Taken from http://docs.python.org/2/library/rlcompleter.html
    print("Jedi is not installed, falling back to readline")
    try:
        import readline
        import rlcompleter
        readline.parse_and_bind("tab: complete")
    except ImportError:
        print("Readline is not installed either. No tab completion is enabled.")
```