init & current working directory

This PR adresses issue #336.

It fixes an issue with the `bin/atoum` script, which when being symlinked would create default configuration and bootstrap files in the directory where the link's target is. 

Example with composer:

````
$ tree .
.
├── bin/
│   └── atoum => vendor/atoum/atoum/bin/atoum
├── src/
└── vendor/
    └── atoum/
        └── atoum/
            └── bin/
                └── atoum
```

Running `bin/atoum` from the project root directory will produce:

````
$ tree .
.
├── bin/
│   └── atoum => vendor/atoum/atoum/bin/atoum
├── src/
└── vendor/
    └── atoum/
        └── atoum/
            └── bin/
                ├── atoum
                ├── .atoum.php
                └── .bootstrap.atoum.php
```

This is not what we want. With this patch, the symlink path will not be expanded so when running the same command, we get:

````
$ tree .
.
├── bin/
│   ├── atoum => vendor/atoum/atoum/bin/atoum
│   ├── .atoum.php
│   └── .bootstrap.atoum.php
├── src/
└── vendor/
    └── atoum/
        └── atoum/
            └── bin/
                └── atoum
```

The `--init` argument now takes an optional value letting the user initialize any directory. This is really useful for those installing atoum globally.