Add subcommand CLI.

Remove `urbansim_compile` and `urbansim_serve` and replace them
with a top level `urbansim` command that takes subcommands.
Currently has `compile` and `serve` commands.

Examples:

```bash
$ urbansim compile configs/*.yaml
```

```bash
$ urbansim serve data/data.h5
```

The interface is built using cliff: https://cliff.readthedocs.org
I added that to the install requirements along with bottle, which
is required for serve. As another project I'll move the server
dependencies out to an extra_requires argument so they aren't installed
for pure modeling.

@fscottfoti, you should pull this and make sure the serve command still works.