Added a Thorfile with a command to easily create boxes.

I've been building some custom definitions based on the ones in bento and found myself being a little repetitive when building and exporting the boxes, so I've written a simple Thorfile that provides a `create` command that can build, export and optionally add a box to vagrant.

A simple command like this will build a box and export it
```
$ thor bento:box create debian-7.1.0
```
Adding `-a` will also add it to vagrant
```
$ thor bento:box create -a debian-7.1.0
```
Adding `-ac` will add it to vagrant and delete the exported file
```
$ thor bento:box create -ac debian-7.1.0
```
And my favorite:
```
$ echo centos-6.4 debian-7.1.0 | tr ' ' '\n' | parallel --delay 10 thor bento:box create -fac {}
```
If you have [GNU Parallel](http://www.gnu.org/software/parallel/) installed, the above command will create the two boxes in parallel.

For more help and examples, run:
```
$ thor bento:box help create
```

I've tested on virtualbox and vmware_fusion providers. I haven't tested kvm but that should work as well. I'm not sure about vmware_workstation, do veewee 'vmfusion' boxes work on vmware_workstation?