rsshell "teleports" to ROOT

`rsshell` now sets the working directory to `$RED_SPIDER_ROOT`. This makes sense, and is convenient, because the user likely wants to be there. Upon `exit`, `rsshell` resets the working directory to the one prior to invocation.

In the future both behaviours, i.e. the target directory upon invocation and the reset directory upon exit, could be made customizable with command-line arguments, configuration files or both.

In addition the introduction message has been made more informative.

Before the changes can be merged the following needs to be tested.

On Linux:

```
 1. cd $RED_SPIDER_ROOT

 2. # if you didn't already have me:
    git remote add julian git://github.com/jgonggrijp/the-red-spider-project.git

 3. git fetch julian
    git checkout julian/rsshell-workdir
    ./setup.py
    # follow the instructions, say "yes" when it asks whether you want to install

 4. # if one of the below commands yields an empty string,
    # log out of your computer and log back in
    echo $RED_SPIDER_ROOT
    which rsshell
    # the latter should be $RED_SPIDER_ROOT/extbin/rsshell

 5. cd /very/far/away  # preferably a subdirectory of a different volume
    rsshell
    # you should get an introduction message with four parts:
    # "Welcome ...", "RED_SPIDER_ROOT = ...", "... teleported ...", "... exit ..."
    # and then see the shell prompt.

 6. pwd
    # should confirm that you're indeed in $RED_SPIDER_ROOT/work

 7. exit
    pwd
    # you should now be back in /very/far/away from step 5

 8. rsshell xkcd-fetch -l
    # should produce the metadata of the latest comic
```

On Windows:

```

 1. cd %RED_SPIDER_ROOT%

 2. # if you didn't already have me:
    git remote add julian git://github.com/jgonggrijp/the-red-spider-project.git

 3. git fetch julian
    git checkout julian/rsshell-workdir
    setup.py
    # follow the instructions, say "yes" when it asks whether you want to install

 4. # if one of the below commands doesn't produce a concrete path,
    # log out of your computer and log back in
    echo %RED_SPIDER_ROOT%
    where rsshell
    # the latter should be %RED_SPIDER_ROOT%\extbin\rsshell

 5. cd /d x:\very\far\away  # preferably a subdirectory of a different drive
                            # leave out the /d switch if there's no other drive
    rsshell
    # you should get an introduction message with four parts:
    # "Welcome ...", "RED_SPIDER_ROOT = ...", "... teleported ...", "... exit ..."
    # and then see the cmd prompt.

 6. cd
    # should confirm that you're indeed in %RED_SPIDER_ROOT%\work

 7. exit
    cd
    # you should now be back in x:\very\far\away from step 5

 8. rsshell xkcd-fetch -l
    # should produce the metadata of the latest comic
```