Fixes #81 (Internal error thrown by image slice)

This pull request adds code to the image slice's RESTful API that properly handles `ProjectHanlon::Error::Slice::MissingArgument` errors that are returned from the underlying codebase. These errors are thrown when either the name or the version for an image (or both) are not included as part of a request to add an `os` type image. Without these changes, the user is presented with a simple "Internal Error" on the command line that is less than helpful (especially when using the command-line interface, because a stack trace was not returned there). With these changes, any `ProjectHanlon::Error::Slice::MissingArgument` errors are caught in the RESTful API code and transformed to `Net::HTTPBadRequest` responses, so that the result can be parsed and a meaningful error returned to the user.

The effects of this pull request can be seen here.  First, this is the error seen when a name is not included on the CLI and you are attempting to add an `os` image but have not provided a name (without these changes):

```
# hanlon image add -p ~/CentOS-6.4-x86_64-minimal.iso -t os -v 6.4
Attempting to add, please wait...
[image] [add_image] <-Internal Server Error

Command help:
hanlon image add (options...)
    -t, --type TYPE                  The type of image (mk, os, esxi, or xenserver)
    -p, --path /path/to/iso          The local path to the image ISO
    -n, --name IMAGE_NAME            The logical name to use (os images only)
    -v, --version VERSION            The version to use (os images only)
    -h, --help                       Display this screen.
# 
```

and this screenshot shows the result returned via the RESTful API without these changes:

![error-return-before](https://cloud.githubusercontent.com/assets/1375734/3340602/7e0d8e4c-f87f-11e3-85a3-9dfb03a844f1.png)

while this is how the error appears on the command line after these changes have been applied:

```
 # hanlon image add -p ~/CentOS-6.4-x86_64-minimal.iso -t os -v 6.4
Attempting to add, please wait...
[image] [add_image] <-image name must be included for OS images

Command help:
hanlon image add (options...)
    -t, --type TYPE                  The type of image (mk, os, esxi, or xenserver)
    -p, --path /path/to/iso          The local path to the image ISO
    -n, --name IMAGE_NAME            The logical name to use (os images only)
    -v, --version VERSION            The version to use (os images only)
    -h, --help                       Display this screen.
# 
```

and this screenshot shows the result returned from the RESTful API after these changes have been applied:

![error-return-after](https://cloud.githubusercontent.com/assets/1375734/3340648/0e380a6a-f880-11e3-9124-4c3dddb1b5b1.png)

Obviously, the changes in this pull request make the response much easier for an end-user to understand.