(maint) Refactor subcommands and support retrieval of multiple vms

This commit refactors the subcommand layout to the layout prescribed by
the `go` tool which should make man pages a little easier to navigate.
This commit also adds functionality to support retrieval of multiple vms
using a space delimited list of pool names after the grab command.
```
vmpool grap pool1 pool2 ...
```

Other implementations of the grabbing of multiple vms could be
`grab -3 debian-7-x86_64` or `grab debian-7-x86_64 -3` but that will require more significant code changes. Another useful feature would be to delete multiple vms which isn't supported by the api but one could easily do here.

This PR also adds the ability to delete multiple vms at once via delete, for example:
```
vmpool delete host1 host2 ...
```