remove all available versions of a box

This patch introduces a new parameter --all for the remove command of the box plugin.

While using the parameter --all when removing a box will remove all available versions of the box.

Example usage:

```
berendt/ubuntu-14.04-amd64          (virtualbox, 0.2.11)
berendt/ubuntu-14.04-amd64          (virtualbox, 0.2.12)
berendt/ubuntu-14.04-amd64          (virtualbox, 0.2.13)
```

```
You requested to remove the box 'berendt/ubuntu-14.04-amd64' with provider
'virtualbox'. This box has multiple versions. You must
explicitly specify which version you want to remove with
the `--box-version` flag.

Versions:
0.2.13, 0.2.12,  0.2.11
```

Using the --all parameter now it's possible to remove all versions at once.

```
Removing box 'berendt/ubuntu-14.04-amd64' with provider 'virtualbox' in version '0.2.13'...
Removing box 'berendt/ubuntu-14.04-amd64' with provider 'virtualbox' in version '0.2.12'...
Removing box 'berendt/ubuntu-14.04-amd64' with provider 'virtualbox' in version '0.2.11'...
```