Re-enable setup.py options but keep setup.py develop as default

Fixes #1042.

I personally agree that the current restrictions on running `setup.py` might be slightly too stringent and might be frustrating for power users who know what they are doing, and are forced to edit the source code to be able to run `setup.py install`.

This PR proposes an alternative approach. All commands are enabled by default, except the `install` command. Running this command will result in the following message:

```
Because Pylearn2 is under heavy development, we generally do not advice using
the `setup.py install` command. Please consider using the `setup.py develop`
command instead for the following reasons:

1. Using `setup.py install` creates a copy of the Pylearn2 source code in your
Python installation path. In order to update Pylearn2 afterwards you will need
to rerun `setup.py install` (!). Simply using `git pull` to update your local
copy of Pylearn2 code will not suffice.

2. When using `sudo` to install Pylearn2, all files, including the tutorials,
will be copied to a directory owned by root. Not only is running tutorials as
root unsafe, it also means that all Pylearn2-related environment variables
which were defined for the user will be unavailable.

Pressing enter will continue the installation of Pylearn2 in `develop` mode
instead. Note that this means that you need to keep this folder with the
Pylearn2 code in its current location. If you know what you are doing, and are
very sure that you want to install Pylearn2 using the `install` command
instead, please type `install`.

Installation mode: [develop]/install/cancel: 
```

I assume that most novice users (for whom the current restrictions were put in place) will simply press enter, while power users can enter `install` if they want/need to for some reason.