GitHubEnterprise: Add url attribute; show in _repr

Without this PR, it's not easy to get the `url` from a `GitHubEnterprise` object. With it, there is a URL attribute. Also the output of `_repr` has been improved.

#### Without this PR:

```
In [1]: import github3

In [2]: github = github3.GitHubEnterprise('http://github.mycompany.com')

In [3]: github
Out[3]: <GitHub Enterprise [0.url]>
```

The `"0.url"` in the above output isn't terribly useful. It looks like an attempt was made to show the URL, but it got messed up.

#### With this PR:

```
In [1]: import github3

In [2]: github = github3.GitHubEnterprise('http://github.mycompany.com')

In [3]: github
Out[3]: <GitHub Enterprise [http://github.mycompany.com]>
```