expose elasticgit.tools as a command line program

```bash
$ eg-tools --help
usage: eg-tools [-h] {dump-schema,load-schema,migrate-gitmodel-repo,shell} ...

Elasticgit command line tools.

positional arguments:
  {dump-schema,load-schema,migrate-gitmodel-repo,shell}
                        Commands
    dump-schema         Dump model information as an Avro schema.
    load-schema         Dump an Avro schema as an Elasticgit model.
    migrate-gitmodel-repo
                        Migrate a GitModel based repository layout to
                        anElastic-Git repository layout
    shell               Load a repo and make an EG workspace available for
                        debugging

optional arguments:
  -h, --help            show this help message and exit
```